boiler locks out after running for approx 10 mins?

i checked a boiler for a friend, it fired up, run for approx 10 mins then locks out & the pilot light goes out. there's an electrode with a wire connected to it & going directly to the circuit board i'm assuming it's possible it's the gas valve that's faulty as there's power from the circuit board to the gas valve. the gas valve has a double solanoid. it is a conventional y plan potterton profile boiler. there's no timer or room stat controls apart from a control stat operated maually on the boiler itself. i've suggested it would be more economical to have a condensing boiler fitted but they just want a tempory solution till the new year, the don't want to feel they're going to spend too much on a system with is eventually going to be changed.