Found this page on my dead PSU https://pa0fri.home.xs4all.nl/Diversen/EP925/ep925eng.htm which makes it sound like one of these beefy 2N3055 transistors has probably died.
No sign of scorch marks etc though! 
I still think dumping the full 24 volts across the terminals is a rubbish failure mode.
Dead transistor now identified!
Attached is how my cheapo component tester said it behaves now. Basically a short from collector to emitter, and that’s how I found it (meter on continuity mode, testing across disconnected transistors).
Tempted to replace all of them as they’re not too expensive.
@m0puh
Have you done any more poking around?
@ab3xu I’ve taken the lid off and I’m currently trying to get to the pass transistors so I can see if any are dead. It’s a bit like taking a 3D jigsaw apart!
@m0puh fun times!
@ab3xu Yep! Found the dead transistor though, now I just need to order a new one 
@m0puh Fantastic! Though shipping will probably cost more than the transistor.
@ab3xu they're about £1 each, so I think I'll replace all of them and get some spares. Amazingly, shipping is free from CPC.
Of course once I get them installed I'll probably find it wasn't the only problem 
@m0puh
One step at a time when you have to order for each step is a real bother.
Way back I worked as a repair tech, it was so nice to have replacements for every part on the board on hand and neatly arranged in bins.
@ab3xu I bet, it would be great to have everything I needed to fix this in one session.