So talks an expert - didn't Homer Simpson use a nodding bird to type Y on his keyboard when he was too fat to go to work?
You might consider, next time it fails, taking off the outer cover and tapping around with a wooden spoon - the fault symptom you describe strongly suggests a poor electrical connection or a dry joint on the control board - if you can find it with a bit of selective vibration you might save yourself £££. Once you find the problem it may well be a simple fix.
Do not let people on here frighten you by saying you should be Gas Safe Registered to do this - The Gas Safety (Intallation & Use) Regulations 1998 sepecifically say...
(6) Nothing in these Regulations shall apply in relation to
(c) work in relation to a control device on a gas appliance if -
(i) the device is intended primarily for use by a consumer of gas; and
(ii) the work does not involve breaking into a gasway.
Good luck...
Peccavi, July 2009