Fixed! :-)
Where the front display/ control panel meets the main amp/tuner panel
there are a series of wire links. Helpfully labellled too. Touching
the L-in and L-out with a finger brought full audio. Same for R
channel.

After much head scratching as to where the signal was getting lost, I
found a hairline crack acorss the pcb behind the EQ/treble sliders.
Repairing this fixed the unit.

regards, B