Oh FFS ... now what ?!

Try a different PSU. I have many SPA3000's around the family, 13 years old. I think all the original PSU's have given out in that time. It's the output smoothing capacitor which dies. Test it by measuring the output voltage with no load, and then with about 50% rated load. If the output capacitor has died, the voltage will drop considerably when a load is applied. If you have a scope, you will also see the output is extremely noisy with a load.

Conversely, only a single SPA3000 has died in 13 years.

I had a fax machine do this once, it was during a thunderstorm and Samsung originally tried to refer me to my house insurance, but after some research I found an awful lot of the batch I had had done the same apparently randomly, so I persisted and a bloke came and fitted a new motherboard telling me Samsung had fitted a batch of chips unusually sensitive to static discharge. this was of course a long while ago in the80s. In your case though, it sounds like its an old unit. Maybe it has been trashed by a psu failure, might be worth changing the psu as well when you get the next one from the loft. Brian