Dell systems are well known to be picky about RAM compatibility. Crucial RAM is the most compatible and most recommended for Dell systems. Using other brands is a hit and miss proposition. The specs call for DDR4 3200 UDIMM and for 64GB total that would be slots UDIMM 1 and UDIMM2. But I am guessing you already knew that. 

Thanks. Was installed as 2x32Gb of DDR4 3200Mhz in slots 1&2.

Have ordered Crucial now, but it's only CL22.

You implied you bought XE4 SFF, because SFF uses ddr4 while XE4 tower uses DDR5 memory.  When you say you spec’d cheapest, do you imply 1x4 GB ddr4?

XE4 SFF memory config:

4GB, 1 x 4GB, DDR4, 3200MHz, single-channel 8GB, 1 x 8GB, DDR4, 3200MHz, single-channel 8GB, 2 x 4GB, DDR4, 3200MHz, dual-channel 16GB, 1 x 16GB, DDR4, 3200MHz, single-channel 16GB, 2 x 8GB, DDR4, 3200MHz, dual-channel 16GB, 4 x 4GB, DDR4, 3200MHz, dual-channel 32GB, 1 x 32GB, DDR4, 3200MHz, single-channel 32GB, 2 x 16GB, DDR4, 3200MHz, dual-channel 32GB, 4 x 8GB, DDR4, 3200MHz, dual-channel 64GB, 2 x 32GB, DDR4, 3200MHz, dual-channel 64GB, 4 x 16GB, DDR4, 3200MHz, dual-channel 128GB, 4 x 32GB, DDR4, 3200MHz, dual-channel

You can buy Dell oem ram on eBay for much cheaper price if you can find a deal.  Use the manufacturer model number to search will help.  Dell just buys same memory then does quality control and validation, relabels it and sells at higher price.

This is a user validated 8GB ddr4 for xe4: HMAA1GU6CJR6N-XN 1x8GB

this is the same ddr4 oem ram used in XPS 8940.  Very likely you can pick 2x32 or 4x16 from oem ram list below and search for a good deal on aftermarket.

I found 32gb ddr4 oem ram $69.99 per stick open box.  Is that a good price?  Link provided below.

Dell oem 3200 DDR4 list (XPS 8940):

732YD 32GB DDR4, 3200MHz, 2Rx8, Unbuffered, 288 pin, Dual Rank, 1.2V, Non-ECC, Non-Encrypted, Hynix HMAA4GU6AJR8N-XN/HMAA4GU6CJR8N-XN, Micron MTA16ATF4G64AZ-3G2B1/MTA16ATF4G64AZ-3G2E1, Samsung M378A4G43AB2-CWE

DK8NX 16GB DDR4, 3200MHz, 2Gx64, Unbuffered, 288 pin, Single Rank, 1.2V, Non-ECC, Non-Encrypted Adata AO2P32NCSV1-BEVS, Hynix HMAA2GU6AJR8N-XN/HMAA2GU6CJR8N-XN, Kingston KDK8NX-MIB/KDK8NX-MIE, Micron MTA8ATF2G64AZ-3G2B1/MTA8ATF2G64AZ-3G2E1, Samsung M378A2G43AB3-CWE/M378A2G43BB3-CWE

C5N22 16GB DDR4, 3200MHz, 2Gx64, 2RX8, Unbuffered, 288 pin, Dual Rank, 1.2V, Non-ECC, Non-Encrypted, Adata AO2P32NCST2-BZ7SHD, Hynix HMA82GU6DJR8N-XN, Kingston KC5N22-HYD/KC5N22-MIE, Micron MTA16ATF2G64AZ-3G2J1, Samsung M378A2K43EB1-CWE

9CXF2 8GB DDR4, 3200MHz, 1Gx64, Unbuffered, 288 pin, Single Rank, 1.2V, Non-ECC, Non-Encrypted Adata AO2P32NC8W1-BD3SHC, Hynix HMAA1GU6CJR6N-XN, Kingston K9CXF2-MIB/K9CXF2-MIE, Micron MTA4ATF1G64AZ-3G2B1/MTA4ATF1G64AZ-3G2E1, Samsung M378A1G44AB0-CWE/M378A1G44BB0-CWE

V0M5R 8GB DDR4, 3200MHz, 1Gx64, 1RX8, Unbuffered, 288 pin, Single Rank, 1.2V, Non-ECC, Non-Encrypted, Adata AO2P32NC8T1-BZ5SHD, Hynix HMA81GU6CJR8N-XN/HMA81GU6DJR8N-XN, Kingston KV0M5R-HYD/KV0M5R-MIE, Micron MTA8ATF1G64AZ-3G2J1, Samsung M378A1K43EB2-CWE

Thanks for your post, that's really helpful. Yes SFF (as per the post title).

And spec'd with 4Gb. 64Gb at the time was a £500+ option! But was always planning to increase.

Hadn't really considered eBay. Given I've bought two lots of main brand memory which hasn't worked, but fortunately from Amazon so nice simple returns!

Is the Rx8 bit relevent? Not sure what it means and not always shown on specs.

the seller for open box condition offers 30 day returns | Seller pays for return shipping.  to me it is fair enough and I can trust it.  not sure if 2x$69 for 2x32 gb is a good deal.  does not sound too bad to me.  I would not read too much into memory spec as long as it is from a major maker.  Dell uses a lot of Hynix memory and I trust it has relatively good quality control.

but if you prefer amazon, Amazon memory says it is Hynix compatible.  To me that makes me cringe a little.  I would prefer to buy the original Hynix.  I would make sure with seller what I buy is exactly what is shown in picture.  

SK Hynix HMAA4GU6AJR8N-XN 32GB DDR4 3200Mhz PC4-25600 non-ECC Unbuffered CL22 288-Pin DIMM 1.2V Memory Module for Desktop

Crucial Pro RAM will work just fine... Respect the voltage, and the frequency...