Fixed it
3rd check of the vacuum pipes yielded a split which, when repaired, solved the problem of it falling into limp home in 3rd, 4th, 5th gear at 3k revs. I'm guessing this might have been something to do with the turbo wastegate or something like that. Actually that's a guess, but it's not doing it now.
In other news, maybe it's already noted somewhere on the forum here, but y'all know that if you break down in a (big national chain) supermarket
they have a breakdown service which covers any car in their car park? Just go to customer services, tell them, and a man in a van will come and fix you or tow you to a garage.

Tesco has this & so, I believe do the other big ones.