Final update for the time being as I have to leave soon now:

That short turned out to be intermittent. I hope it was just due to
something shorting out on the bench that won't happen when the casing is
back on because you all know what a bitch it can be to trace intermittent
faults. Anyway, that fault has now disappeared, so I took some voltage
measurements before the 20W resistor got to hot (from 19'C to 60'C takes
about 1.50s now) and I have:

61.7
12.7
5.8
0
-5.8
-12.7
-62.4

This is with the psu board plugged into the scope and all power
connections made except for the VHT stuff.

The correct figures according to the manual should be:

60
12.7
6
0
-6
-12.7
-60

So very close! Looks like the main transformer may be ok after all.