got a call from John at Simplyalfa yesterday and at last they have loacated and fixed the problem. They went through a process of elimination by swapping parts from a good car to mine and vice versa to check as many posibilities as they could and taking diagnostic readings all the time. Eventually they found a wire running from the ECU through the bulkhead to the engine had, over time, rubbed itself bare and caused the problem everytime it came into contact with something. This wire apparently is to do with EGR valve, potentiometer and air mass sensor. I drove home last night in what seemed to be a new car, it pulled like a train again and was a joy to drive. Cant recommend the guys at simplyalfa enough, they kept me up to date all the time, they had the car for just over week, gave me a car off the forecourt as a loaner and only charged me for time fixing the problem, not all the time it took to locate it, or a list of parts fitted that might have fixed the problem ie potentiometer, egr valve etc.
Also thanks to everyone on this site who took time to show an interest,