Hmm. So got it home, opened it up and all connections are solid, clean and as they should be.
Maybe this should have been posted to repairs and technical, sorry.
Took the speaker output from the amp, checked it (visually) cleaned and put it back.
All working, but then it was all working fine until half way through the first set. Only other thing I found was a slightly dirty pot which I cleaned. However I would have thought that ought to cause the sound to deteriorate rather than just cut out.
What else can I check?