Thanks @info. I finally found time to give the device some more attention and the situation hasn’t improved.
Replaced the switch with a fresh soldering job and, with a multimeter, confirmed good contact and that the circuit breaks when the switch is actuated. However when plugged in, the bum key no longer registers at all, even with lots of pressure.
I’m just a humble programmer but my dad the old EE says it’s probably the diode. Are there further diagnostic steps I can take? How would I replace the diode if faulty? Can I seek replacement parts?