Thanks Luke and Wrinx for the replies. I think that it might be the airflow sensor. I took the car to the head mechanic at a local garage (Barry at Coop Alfa in Lincoln). He was really helpful, told me how to strip and clean the airflow box containing the sensor, I'll be doing that next weekend. When I removed one of the pipes from the head to the sensor, it had some oil in it (lots of oil vapours coming from engine). I think that this has coated the sensor. If the strip and clean works, I'll tell you you how easy/blody difficult it was!