Tranny arrived today. I found some spanners big enough to get the old one
out. Fitted the replacement, along with an inrush thermistor from Farnell,
and two new level pots (both the originals went round and round ... ).
Gingerly powered it via the variac and Lo! Up it came !

Which was a relief, because I'm not sure that it looks like one that I would
want to be trying to repair an output stage on, particularly if it was the
lower board that had failed ...

Bucketfuls of watts poured out of both channels, and the fan cut in just
when expected, so I heaved a sigh of relief, screwed the cover back on, and
wrote the ticket out. Then gave myself a hernia getting it off the bench ...
:-)

Arfa