So, the garage have stripped down the engine and disassembled the offending cylinder and discovered that the problem is -wait for it - a broken spring.
The garage is still in negotiations for Peugeot to cover at least some of the cost. Otherwise, this spring - about the size of a little finger - could end up costing us a couple of thousand pounds.
(That's not all for the spring itself. There's also the cost of diagnosing the problem, taking apart the engine and the cylinder, fitting the spring, then putting the car back together again. I suspect the spring itself will be about £30:00.)
I'm wondering if they'll take payment in instalments..?
Whatever else happens, I hope we get our diesel car back soon. I've had enough of stalling the petrol courtesy car and struggling with the feeble 0 - 60 in 3 minutes acceleration afforded by its three cylinder engine.