Update, I took the combo over to a mates house this afternoon, the always lovely @Tmg, with the intention of swapping out the preamp valves, and the amp was not making any strange noises at his place, nor his place of work around the corner, which he assures me has dodgy electrical wiring. We even set it up right next to an old tower PC running 3 monitors and could not get any unhealthy noises out of it. And once back home it was still behaving itself. So, with the lack of interest when I had it up for sale last year for months, plus a possible intermittent issue (although the only place this has happened was at Ainsleys' place as discussed in the original post) I think I might as well hang onto it as it really does sound glorious especially when cranked up.
In the words of my late granddad, Well, bugger me, boy.