Sebi, thanks for the info. I have found that the KA431 is connected to the 5V line, as you thought.
I've now found a couple of things wrong, but won't mention them yet for fear of confusing things even more than they already are. The only thing I will say for now is that I have found the reason for the receiver operating in reverse - power supply on when unit is in standby and power supply off when unit turned on - one of the transistors in the secondary, a big TO-220 package, is completely short-circuit, all three pins short.
I don't know why this is exactly, but have ideas. It may be why all the voltages suddenly became high when they were previously too low, so perhaps not an imperceptible dry-joint after all...
Anyway, this transistor which is short - KSA614-Y - simply marked A614-Y, is to do with the standby operation of the power supply amongst other things, I suppose, as it is connected to a smaller transistor - KSP2222A - which in turn has its base connected to the standby voltage from the mainboard, i.e. power supply turns on when the base has a voltage going to it and goes off when that voltage disappears when the mainboard stops sending it after on/off button has been pressed.
So, I need to replace the KSA614-Y. I don't suppose there is anything that might do on the old chassis I have in the shed from TVs and satellite receivers, etc? Datasheet says it's a low frequency power amplifier, power regulator. I'd like to just test it with anything I can find around that'd do for now until I source a proper replacement, if I can find one.