Not sure what's happened, as I haven't really done anything since the last time, but the "c" and "v" keys now appear to be working fine.
"m" and "n" are still as they were... so perhaps it's a dry solder joint, or something shorting somewhere... I'm thinking...
The other issue with the left mouse button is also odd. It works fine in games, and when I press it in workbench, it immediately works to pull the workbench window up or down, but doesn't work to interact with anything (ie. clicking on icons). Pressing the "Alt" key appears to issue a left mouse button click...
Could it be that one of the Amiga key (signals) is stuck on? I think the shortcut for the left mouse button should be Amiga + Left Alt, is this correct?
Incidentally I ordered a replacement power supply from a guy in Poland via Ebay, although they also have a website here: https://www.c64psu.com/
I haven't looked inside it yet, but the voltages were good, and it powered up the A1200 just fine
Might take a look at its insides over the weekend now I've confirmed it's working.