Ah, the final chapter in the repair log..

Buzzed out PSU with DMM from PSU side of fuse socket to N side of IEC inlet;
470KOhms, rising to a few megohms after a few seconds.

Replaced mains inlet fuse (2.5A 20mm antisurge/slow-blow) this morning,
powered on to full white screen with no retrace lines. Connected VGA tester,
loud "thunk" sound, followed by black screen and good image. Closed cover, put
back into service this afternoon.

Total cost: a 20p fuse. Some repair...

--
Phil.