Turns out it was the lambda sensor... but while it was in I had a 72k service and the belts and tensioners changed. It's only been about 35k since they were last done, but I'm feeling very lucky now - all three tensioners were shot - two of them were very noisy with the bearings sounding like they were about to fall out and the cambelt tensioner, even though it was a newer 'white' type, was well worn. All in, it cost me a gnats chuff over £500, but I reckon it wsa money well spent... it's running a lot better and the engine is so much quieter!!
SF - the MOT tester didn't test the emissions before the cat - he said that even without a cat, the readings shouldn't be that high - i.e., the cat doesn't remove that much anyway. But, lambda it was, and the CO reading is down from 5.38% to 0.25%. I should recoup some of the above bill on reduced fuel costs!