SO as an update (I know you've all been on the edge of your seats). I opened it up last night and wailed on it for a bit until it did the feedback, then started with the tapping.
Preamp valves - went through all of them, no noticable sounds from the amp.
So I set about on the power amp valves - got to the 2nd one and gave it a tap - all of a sudden the ringing stopped. Hm, that's interesting I DID THINK TO MYSELF.
Maybe the poweramp tube was a bit loose and I tapped it back into place? Seems weird but I don't have much more of an explanation.
Put volume and master on 10 and wailed like killmeister for a couple of minutes, threw the fuzz into the mix as well. Then I STFU'd and there was no feedbacking.
Success? maybe so