Okay, regulator was faulty. At the same time, this doesn't explain the engine cuts as the mags are on a separate circuit from battery and Alternator (which is as it should be).
Best thinking on that is that they choked up after a long period at altitude unleaned, hence the need to add power to keep everything running.
When I cut the throttle to land I was expecting the engine to stop - and it did.
So, I think we have it.
Not that I'm usually flying these sorts of aircraft that high for that long - but some form of leaning would seem a good idea!
Cheers, Sam.