Baxi Bermuda problems

We have a Baxi Bermuda 50/4E back boiler and Bermuda "Inset 2 Super" gas fire. All electronic.
This has been installed for us about 5 years by CORGI engineer and it is the first problem we have had.
When cool, the boiler lights first time with the reassuring "whoosh" that we have grown used to but after about 10 minutes it cuts out and then repeatedly tries to reignite, fails, then tries again etc. The sound is just that of a flame being blown out or running out of fuel, if you know what I mean! Left for another 10 mins to cool, the boiler will re-fire and run for, say, another 10 mins then the problem starts again.
Hot water and heating are both affected. The motorised valves are OK as I can get either/both hot water and/or heating from the boiler whilst it is running. I have also purged air from the radiators and also from the stub by the pump (which by the vibration and temperature and also rads getting warm would indicate that the pump is working OK) to no avail.
It was suggested to me by a friend that the "Gas valve" has failed. This seems unlikely as I can hear our gas meter making the normal (money gobbling) noise when gas flows each time it tries to ignite. Between attempts, the meter is quiet. This would suggest to me that the gas valve is OK.
I am not going to tinker with a gas appliance, unless this is a simple well known problem that does not require anything other than a part that can be changed by me without any problems and I doubt very much that will be the case :-(
What would the normal cause of this problem be?
How much would a good engineer charge?
Any info or advice very gladly received before calling people out!
Thanks Steve
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