Pilot light keeps going out?

I have a potterton netaheat electronic Ep2000 boiler, the pilot light keeps going out, (it will ignite when it first comes on, but after that there is a spark but does not ignite) therefore I get no hot water or radiators, my gas man came out and had a look and charged me £47.00 but the problem is still there and I cannot get hold of him!!!
Would I need a new thermal couple? Is this a common fault?