I have a similar story to that Paul, I was racing in 1979 using a Honda, it was a two day meet at Snetterton in Norfolk about 31/2 hours drive away, (with caravan) on the Sat morning, I practised and didnt make the fourth lap, in those days three was compulsory for a start in the race to be permitted (dont know about now) and the engine seized solid, I was trailered back to the paddock, and we took a quick look, everything externally looked hunky dory, it didnt free on cooling, nothing showed through any of the eight tappet covers, so we loaded up and headed for home, this was before lunchtime, about half way home a pal who was coming to watch, aproached coming the other way, spotted the Land rRover and a s soon as we passed we recognised each other, I saw his brake lights come on so stopped, he returned and we chatted, the long and the short of it was that he would meet us back at home (he was on a similar bike) and we would see if we could do anything, later that PM at home we looked at everything possible, and failed to find it, so we took the motor out and stripped it progressively thinking we would find it, but failed again, and (as you) it was stripped to its last nut, so we started to reassemble it again thinking that the fault would show on the re-assembly, but it didnt, we had no new gaskets so used all the old ones, (I had previously had it apart and greased all the new gaskets) didnt find anything wrong, continually turning the motor as each part was added, nothing found, finally re-adjusted the tappets, set the timing and fired it up, it ran well, balanced the carbs and at 2am Sunday put it back in the van, and got a few hours kip, drove back up to Snetterton (I was sponsored at that time for petrol costs) and got in two races, due to the fact that I had got those three practice laps in, it never faltered and I rode it for the rest of the season, buying a Monty in the Winter ready for the following March. We never knew what it was, never saw anything physical and could not elieve that we missed whatever it was, very strange, but locked solid it was.
I wish I had half that energy now.
CLEM