Ok, quick update. Visited the garage on my way home today and chatted with Martin who is working on my car (he's one of the owners of Apple MG), and also had a look at the car.
Initial tests showed no compression on 3 out of 4 cylinders. With the head off, there's no apparent damage to the head/valves etc, no apparent piston damage, but fluid poured into the cylinders disappears in about a minute. From this, the suspected cause would be bore wash from overfuelling. T4 (Testbook version 4.Computer Diagnostic System)
fault codes are showing on sensors (cam, crank, temp), and the timing also seems to be slightly out, which shouldn't happen with a chain. They set the engine to TDC before they took the head off, but the pistons were slightly off TDC positions - almost like when a cambelt has jumped a tooth or two. So it's all still a bit of a mystery.
The good news is the replacement engine has arrived safely, and fitting begins tomorrow. The new engine includes everything except the starter, alternator and gearbox, so I'll have a nice box of spares (hoses, injectors, etc) to take away with me once they are done. The garage will replace all filters, fluids etc, and also fit a new OEM thermostat - nice and easy to do with the engine out of the car!
Apple MG seem very knowledgeable and I'm confident that my car is in very, very good hands. I should have my baby back early next week, and in even better condition than it was before the breakdown!