Central Heating will not come on Ravenheat CS1 85T Boiler?

When I switch to central heating on Ravenherat CS1 85T Boiler it doesn't fire up but when I switch to hot water it does fire up and water is hot but when I turn over to central heating the radiator heats slightly and then after a minute it goes off. Just had new Valve put in as hot water wouldn't come on and red light kept showing but there is no red light showing this time. I thought it might ber because the pipes may be frozen so ran hot water and put it on max as today is a freezing cold day I haven't had it serviced for 2 years could it be this. Had no luck with this boiler since it was installed in April 2005. I would be very grateful if anyone could help. Thank-you.