I can probably guess why the block is warped too. When I got the car, the water pump and the radiator were both leaking slightly, and the cooling system was half empty. I made sure I kept the water topped up on the way home, but if the previous owner had at any point allowed the engine to boil dry, that could easily have caused the problem.
I didn't buy the rebored block that was on eBay - it went for too much money, considering it was a bit of an unknown quantity and it probably would have cost another £50 or so to transport it up to me. So this week I need to look into quotes for having Lintilla's block rebored and the top surface skimmed.
Fenchurch is going in for her MOT tomorrow. Fingers crossed that she will pass first time, because with Lintilla off the road again I'm back to commuting on the bus if Fenchurch is out of action too.