Opened it up to see if the earthing strap or something obvious like that was hanging off, apparently common with amps like mine, but everything seemed to be connected securely.
As a last resort decided to lift the circuit boards out to look for anything nasty underneath, saw some ugly but probably ok soldering on the main amplification circuit, all the front panel stuff looked fine.
So I cleaned all the boards with some of that electronics solvent stuff and a toothbrush to get off the little bits of gunk off and re-soldered all the joints that I didn't like looks of put it back together and later on plugged it in to test.
At first I thought that I had completely broken it due to lack of noise of any sort, but when I cranked everything up full on it there was a very slight static noise coming from the speakers, that was promising, time to turn everything down and put a bass in.
Plugged in a Bass and hit a string, and a nice loud crisp twang came out of the speakers, I think it is better sounding now than it ever was.