Turns out to be scale in the gas valve blocking the filter!
had the meter changed anout 9 mths ago & guess this dislodged a load of bits which have built up over time. Hot water at last cheers all :P
I have an 18 mth old Baxi combi, just serviced, but when you call for hot water it cycles hot then cold. Burners yellow light comes on for about 20s the goes off for 20s etc. The water has been getting progressively cooler over the past few weeks, but put it down to the cold weather. Any Ideas?