due to it running lean, particularly in the low revs / on closed throttle..
due to either the mixture being off (lambda or something, but then it'd be really screwed wouldn't it?), or the presence of an "air leak".
Now if its the second thing --- 1. what is he on about? 2. where would this be occurring (so i can take a stab at whether it might be my own fault)?? 3. how in the world might i fix it???
cheers guys+gals
(The engine otherwise seems fine - there was a scummy layer of oil and rust on the top of the coolant in the expansion tank when i went to top it off today, but hardly anything - there was some spillage and contamination when i was cleaning anyway so that should be all of it - no mayo in the cap, pulls nicely for a half-throttle low-revs 1 litre, runs as smooth and quiet as it ever managed to before, not overheating, not chowing fuel, oil or coolant, etc... yay!)
other than that and a busted side repeater (neither being a fail condition) and noting of the small rust patches on the arches, it not so much flew thru the MOT as attached itself to a JATO and launched into space. Go polo!
... from what i've seen during washing the both today, in 18 months the astra will probably have as much corrosion.