Update on the progress so far.
Thank you everyone for your input and the answer to the various suggestions is as follows.
The power amp does indeed have its power rails intact and the speaker protection relay is working just fine. I had to move it off the bench in order to tear down my latest acquisition, a Tektronix TDS210 DSO and check the state of the power supply for bad caps etc and it was all fine. This is back together and working.
Popping the Kenwood back on the bench and plugging in some headphones and switching, and hey presto the tuner was working and it seemed so was everything else, I connected a meter to the speaker terminals, removed headphones and the speaker terminals were powered up just fine.
I then set about poking here and there and twisting the chassis and it went dead, then came back to life and went off again, this was when I was moving the audio input boards and also the video boards. It seems that these being plugged into each other via extended header pins, these also plug into the main amp via more edge plug/sockets so the problem appears to more a bad contact on one or more of these plug/socket interfaces.
So now I'll have disassemble this section and do a though clean up of connections and then assemble and check again.
When I've done this I'll come with a further progress report but in the meantime I'm still looking for a decent copy of the service manual for this if anyone could assist here it would be a great help as the ones on the internet for down don't tell the whole story and the schematics have junks along the extreme edges of the pages chopped off in the original scanning process.