Just spoken to uncle Nelly who's managed to unearth the root of the problem with my DD engine.
Turns out the generator was loose, creating enough heat to boil the oil and melt any aluminium components within range of the magnets/windings. Apparently it's a real horror story, with that side of the engine full of molten blobs of alloy and burnt oil ("looks like armageddon in there, worst case I've ever seen"!).
This would explain the heat being generated in the bores as the oil wouldn't be up to the job of lubricating at the time the engine needs it most. It'd also explain the nasty, nasty noises the engine was making when I tried to start it up again.
Not sure what we're going to do but Nelly's investigating a reasonable-cost option to put it back together and see if it holds.
Ho-hum.
ali