Dell Optiplex 790 light

Doesn’t explain the PSU light though?

Check the manual-- OptiPlex 790 Owner's Manual (Desktop) (dell.com)

flashing amber and blank screen is no POST.

try replace cmos battery w a brand new CR2032 and clear motherboard cmos settings by RTCRST jumper.

to test psu try Dell BIST.  How to Run a Power Supply Unit Self Test On a Dell Desktop or All-in-One Computer | Dell US

When the pc is on, amber flashes on power button, dvd drive flashes green, number 1 & 3 flash and the PSU green light rapidly flashes. When I hold the test button it shuts the pc down?, it also clicks. 

I’m out of pocket £70 if I can’t get this machine up and running again so I need it to work as soon as so I can sell it on again 

Go with @redxps630 suggestions first and replace the size 2032 CMOS battery.  Bad batteries have been known to throw bad error codes.

The error code presented isn't in the manual with amber power light @Mary G .  If still no luck, reseat RAM one stick at a time, powering on after each one.  See if it boots or error code remains the same.

Re: When the pc is on, amber flashes on power button, When I hold the test button it shuts the pc down?, it also clicks. 

these make me wonder if the psu is bad.  if you have a spare working 24+4 pin psu use it to test your 790 motherboard.

Also note if I go with https://www.dell.com/support/kbdoc/en-us/000126021/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators#2009_to_2012 :

...

Got past the amber light warning, it now showing the cpu failure so ended up buying a replacement motherboard which is the same board 

It looks like the pins are bent.