Hi Owen,
I had this exact problem on mine.
Unfortunatly it turned out to be a problem with the ECU, bought second hand ECU and problem sorted.
Try and borrow someones ECU and swop it over to see if it cures the problem. (Located under drivers seat, just unplugs)
If this is the case then either buy second hand (£50 to £100 breakers give about 3 months warranty) or you can send it of to be repaired (£150 to £250).Some ECU repairers will charge £30 to look at it and then give you a quote.
Hope this helps
Simon - Rangie427