On Mon, 17 May 2021 10:19:44 +0100, Peter Able wrote:

snip

After changing the caps and powering it up with no load, I saw a
'weak' (fluctuating) 12V on the DMM but it wouldn't power up with a 5A
load attached (it crowbarred).

snip

There looks to be a preset potentiometer, bottom right, in your photo.
Might be worth noting its current position, rotating it from end to end
of its range a few times, then restoring it to the original position. A
spurt of switch cleaner before the above might help, too.

Good (simple) check / suggestion and nothing to lose. ;-)

I believe that pot is just a trimmer allowing you to tweak the voltage
around the nominal 12V so I don't even have to mark it and would
re-set it under load.

I'm not particularly hopeful it will do anything as even if it was
'noisy' and was allowing the voltage to fluctuate around it's entire
range, I would have thought it should still have been able to drive
the 5A load, even if also whilst fluctuating? I wonder though if that
only depends on if the PSU still works at all with (worst case), no
voltage on the wiper of the trimmer?

I'll let you know how I get on. ;-)

Cheers, T i m