Got back from collecting Prospero, and no, I didn't ask. He's got a new engine, after all. And the vibes are that anyone using a non-VW garage for servicing can whistle for any favours.
There was a very cute white ED35 in the showroom, and the salesman who sold me Prospero saw me in "admire" mode and tried to persuade me it was time for a trade-in! No chance.
Anyway, Prospero is now fine. There was about 45 miles extra on the clock from the breakdown, which doesn't seem unreasonable to me as regards testing. Can't find any fault with the car and actually it's quite hard to get my head round the fact that there's a new engine in there now.
I'm no wiser as to what caused the failure. "Damaged pistons". I get the impression they just looked long enough to realise the engine was buggered, and ordered a new one. I wonder if VW themselves will want to look at the corpse to find out what happened? Of course there's always the matter of the RAC patrolman, who really did flog it while trying to get it started originally. In retrospect, he kept at it beyond the point when it was obviously a dead horse, and there were a couple of quite interesting bangs from the engine during all that. So it might not be entirely straightforward.