Bloody mice!
The air intake above the radiator has a small grille. About the only improvement from standard is to cut that grille out, and previous owners have done that with my CTi. So now...I realise why it was there!
Changed the air filter today, and found the canister choked with leaves, and the rubber and card of the element well chewed; fortunately not right the way through. 115hp + 1 mouse power could have been interesting!
The replacement filter I've bought is a Purflux one, which is sponge on a perforated steel former. Reading the instructions on the box, I see it should be soaked in engine oil, and drained, before fitting. Which I've done...messy...wonder if garages bother with doing that when fitting them.