No replies, and no help from either Dell or the Dell community so I'm glad I found the solution independently. It turns out that if I follow the instructions here:

https://www.mybluelinux.com/debian-linux-and-lsi-megaraid-sas/

using the xenial main (the latest repo supported) in spit of running on focal it installs megacli and then all I needed to realise was that megacli needs to run as root to see the adapter. So:

$ megacli -adpCount
Controller Count: 0.
Exit Code: 0x00
$ sudo megacli -adpCount
Controller Count: 1.
Exit Code: 0x01

 And so:

$ sudo megaclisas-status
-- Controller information --
-- ID | H/W Model         | RAM    | Temp | BBU    | Firmware     
c0    | PERC H700 Adapter | 512MB  | N/A  | Good   | FW: 12.10.7-0001 

-- Array information --
-- ID | Type   |    Size |  Strpsz |   Flags | DskCache |   Status |  OS Path | CacheCade        |InProgress   
c0u0  | RAID-1 |    232G |   64 KB | ADRA,WB |  Default |  Optimal | /dev/sda | Type : Read Only |None         
c0u1  | RAID-1 |   1818G |   64 KB | ADRA,WB |  Default |  Optimal | /dev/sdb | Type : Read Only |None         

-- Disk information --
-- ID   | Type | Drive Model                                   | Size     | Status          | Speed    | Temp | Slot ID  | LSI ID  
c0u0p0  | HDD  | WD-WCAT1F661705WDC WD2502ABYS-18B7A0 02.03B05 | 232. Gb  | Online, Spun Up | 3.0Gb/s  | N/A  | [32:0]   | 0       
c0u0p1  | HDD  | WD-WCAT1F676236WDC WD2502ABYS-18B7A0 02.03B05 | 232. Gb  | Online, Spun Up | 3.0Gb/s  | N/A  | [32:1]   | 1       
c0u1p0  | HDD  | ST2000DM008-2FR102 0001 ZFL0V6WQ              | 1.818 TB | Online, Spun Up | 6.0Gb/s  | N/A  | [32:2]   | 2       
c0u1p1  | HDD  | ST2000DM008-2FR102 0001 ZFL0V6PW              | 1.818 TB | Online, Spun Up | 6.0Gb/s  | N/A  | [32:3]   | 3       

and

$ sudo megacli -AdpAllInfo -aAll
                                     
Adapter #0

==============================================================================
                    Versions
                ================
Product Name    : PERC H700 Adapter
Serial No       : 06C00DH
FW Package Build: 12.10.7-0001

                    Mfg. Data
                ================
Mfg. Date       : 06/17/10
Rework Date     : 06/17/10
Revision No     : A00
Battery FRU     : N/A

                Image Versions in Flash:
                ================
BIOS Version       : 3.18.00_4.09.05.00_0x0416A000
FW Version         : 2.100.03-4651
Preboot CLI Version: 04.04-010:#%00008
Ctrl-R Version     : 2.02-0025.1
NVDATA Version     : 2.07.03-0003
Boot Block Version : 2.02.00.00-0000
BOOT Version       : 01.250.04.219

                Pending Images in Flash
                ================
None

                PCI Info
                ================
Controller Id	: 0000
Vendor Id       : 1000
Device Id       : 0079
SubVendorId     : 1028
SubDeviceId     : 1f16

Host Interface  : PCIE

Link Speed 	     : 0 
Number of Frontend Port: 0 
Device Interface  : PCIE

Number of Backend Port: 8 
Port  :  Address
0        4433221104000000 
1        4433221105000000 
2        4433221107000000 
3        4433221106000000 
4        0000000000000000 
5        0000000000000000 
6        0000000000000000 
7        0000000000000000 

                HW Configuration
                ================
SAS Address      : 5842b2b016b7c900
BBU              : Present
Alarm            : Absent
NVRAM            : Present
Serial Debugger  : Present
Memory           : Present
Flash            : Present
Memory Size      : 512MB
TPM              : Absent
On board Expander: Absent
Upgrade Key      : Absent
Temperature sensor for ROC    : Absent
Temperature sensor for controller    : Absent


                Settings
                ================
Current Time                     : 11:23:12 1/6, 2022
Predictive Fail Poll Interval    : 300sec
Interrupt Throttle Active Count  : 16
Interrupt Throttle Completion    : 50us
Rebuild Rate                     : 30%
PR Rate                          : 30%
BGI Rate                         : 30%
Check Consistency Rate           : 30%
Reconstruction Rate              : 30%
Cache Flush Interval             : 4s
Max Drives to Spinup at One Time : 4
Delay Among Spinup Groups        : 12s
Physical Drive Coercion Mode     : 128MB
Cluster Mode                     : Disabled
Alarm                            : Disabled
Auto Rebuild                     : Enabled
Battery Warning                  : Enabled
Ecc Bucket Size                  : 255
Ecc Bucket Leak Rate             : 240 Minutes
Restore HotSpare on Insertion    : Disabled
Expose Enclosure Devices         : Disabled
Maintain PD Fail History         : Disabled
Host Request Reordering          : Enabled
Auto Detect BackPlane Enabled    : SGPIO/i2c SEP
Load Balance Mode                : Auto
Use FDE Only                     : Yes
Security Key Assigned            : No
Security Key Failed              : No
Security Key Not Backedup        : No
Default LD PowerSave Policy      : Controller Defined
Maximum number of direct attached drives to spin up in 1 min : 0 
Auto Enhanced Import             : No
Any Offline VD Cache Preserved   : No
Allow Boot with Preserved Cache  : No
Disable Online Controller Reset  : No
PFK in NVRAM                     : No
Use disk activity for locate     : No
POST delay			 : 90 seconds
BIOS Error Handling          	 : Stop On Errors
Current Boot Mode 		  :Normal
                Capabilities
                ================
RAID Level Supported             : RAID0, RAID1, RAID5, RAID6, RAID00, RAID10, RAID50, RAID60, PRL 11, PRL 11 with spanning, PRL11-RLQ0 DDF layout with no span, PRL11-RLQ0 DDF layout with span
Supported Drives                 : SAS, SATA

Allowed Mixing:

Mix in Enclosure Allowed

                Status
                ================
ECC Bucket Count                 : 0

                Limitations
                ================
Max Arms Per VD          : 32 
Max Spans Per VD         : 8 
Max Arrays               : 128 
Max Number of VDs        : 64 
Max Parallel Commands    : 1008 
Max SGE Count            : 60 
Max Data Transfer Size   : 8192 sectors 
Max Strips PerIO         : 42 
Max LD per array         : 16 
Min Strip Size           : 8 KB
Max Strip Size           : 1.0 MB
Max Configurable CacheCade Size: 0 GB
Current Size of CacheCade      : 0 GB
Current Size of FW Cache       : 394 MB

                Device Present
                ================
Virtual Drives    : 2 
  Degraded        : 0 
  Offline         : 0 
Physical Devices  : 5 
  Disks           : 4 
  Critical Disks  : 0 
  Failed Disks    : 0 

                Supported Adapter Operations
                ================
Rebuild Rate                    : Yes
CC Rate                         : Yes
BGI Rate                        : Yes
Reconstruct Rate                : Yes
Patrol Read Rate                : Yes
Alarm Control                   : Yes
Cluster Support                 : No
BBU                             : Yes
Spanning                        : Yes
Dedicated Hot Spare             : Yes
Revertible Hot Spares           : Yes
Foreign Config Import           : Yes
Self Diagnostic                 : Yes
Allow Mixed Redundancy on Array : No
Global Hot Spares               : Yes
Deny SCSI Passthrough           : No
Deny SMP Passthrough            : No
Deny STP Passthrough            : No
Support Security                : Yes
Snapshot Enabled                : No
Support the OCE without adding drives : Yes
Support PFK                     : No
Support PI                      : No
Support Boot Time PFK Change    : No
Disable Online PFK Change       : No
Support Shield State            : No
Block SSD Write Disk Cache Change: No

                Supported VD Operations
                ================
Read Policy          : Yes
Write Policy         : Yes
IO Policy            : Yes
Access Policy        : Yes
Disk Cache Policy    : Yes
Reconstruction       : Yes
Deny Locate          : No
Deny CC              : No
Allow Ctrl Encryption: No
Enable LDBBM         : Yes
Support Breakmirror  : No
Power Savings        : No

                Supported PD Operations
                ================
Force Online                            : Yes
Force Offline                           : Yes
Force Rebuild                           : Yes
Deny Force Failed                       : No
Deny Force Good/Bad                     : No
Deny Missing Replace                    : No
Deny Clear                              : No
Deny Locate                             : No
Support Temperature                     : No
NCQ                                     : No
Disable Copyback                        : No
Enable JBOD                             : No
Enable Copyback on SMART                : No
Enable Copyback to SSD on SMART Error   : No
Enable SSD Patrol Read                  : No
PR Correct Unconfigured Areas           : Yes
Enable Spin Down of UnConfigured Drives : No
Disable Spin Down of hot spares         : Yes
Spin Down time                          : 30 
T10 Power State                         : No
                Error Counters
                ================
Memory Correctable Errors   : 0 
Memory Uncorrectable Errors : 0 

                Cluster Information
                ================
Cluster Permitted     : No
Cluster Active        : No

                Default Settings
                ================
Phy Polarity                     : 0 
Phy PolaritySplit                : 0 
Background Rate                  : 30 
Strip Size                       : 64kB
Flush Time                       : 4 seconds
Write Policy                     : WB
Read Policy                      : Adaptive
Cache When BBU Bad               : Disabled
Cached IO                        : No
SMART Mode                       : Mode 6
Alarm Disable                    : Yes
Coercion Mode                    : 128MB
ZCR Config                       : Unknown
Dirty LED Shows Drive Activity   : No
BIOS Continue on Error           : 0 
Spin Down Mode                   : None
Allowed Device Type              : SAS/SATA Mix
Allow Mix in Enclosure           : Yes
Allow HDD SAS/SATA Mix in VD     : No
Allow SSD SAS/SATA Mix in VD     : No
Allow HDD/SSD Mix in VD          : No
Allow SATA in Cluster            : No
Max Chained Enclosures           : 1 
Disable Ctrl-R                   : No
Enable Web BIOS                  : No
Direct PD Mapping                : Yes
BIOS Enumerate VDs               : Yes
Restore Hot Spare on Insertion   : No
Expose Enclosure Devices         : No
Maintain PD Fail History         : No
Disable Puncturing               : No
Zero Based Enclosure Enumeration : Yes
PreBoot CLI Enabled              : No
LED Show Drive Activity          : Yes
Cluster Disable                  : Yes
SAS Disable                      : No
Auto Detect BackPlane Enable     : SGPIO/i2c SEP
Use FDE Only                     : Yes
Enable Led Header                : No
Delay during POST                : 2 
EnableCrashDump                  : No
Disable Online Controller Reset  : No
EnableLDBBM                      : Yes
Un-Certified Hard Disk Drives    : Allow
Treat Single span R1E as R10     : Yes
Max LD per array                 : 16
Power Saving option              : All power saving options are disabled
Default spin down time in minutes: 30 
Enable JBOD                      : No
TTY Log In Flash                 : No
Auto Enhanced Import             : No
BreakMirror RAID Support         : No
Disable Join Mirror              : No
Enable Shield State              : No
Time taken to detect CME         : 60s

Exit Code: 0x00

And all is good. MegaCLI is installed and it works.