How can fix my Vaillant VCWGB 282EH combi boiler?

My vaillant VCW GB 282 EH boiler failed to heat raditors but hot water supply seems to be working. I found it extremely hard from yellow pages who can come and fix this problem(s).
One engineer from a reputable firm came and messed with a
screw driver and failed to mend properly. I dont know whether
this is a ploy to fleece me with charges. Anyone having similar system and with same problem now or in the past and how this was solved may be useful trying to solve this daunting problems.