Hi,
It's probably the PCB which is part no. 245131 which is causing this. Quite often the flow switches also fail, although I have exactly the same issue as you and the flow switches are fine. The PCB is on ebay at £40, which is a lot less than the shops or a plumber! You should be able to fit it yourself if you have the manual, otherwise you can get a corgi engineer to come and do it for you.
Matt E, March 2010