MAF Ma Heid.
A salutary tale - despite the overwhelming advice of the sage forum community, I decided to buy a cheap E-Bay MAF to replace one which, while not making the car drive badly, was throwing up errors on the computer - I've had the yellow dashboard light on since I bought the car in May.
I'd put a bottle of Cataclean through to flush out the Lamda(s), and thought that swopping out the MAF might sort things before winter.
Surprise, surprise, as soon as I turned the car over the idle hunted like mad and then cut out, totally undriveable. Swopped the units back over and Hey Presto, back to normal.
If you're thinking that £30 quid for a new MAF might be worth a punt, take my advice - steer clear.