how can I fix my Myson Midas B combi boiler?

My boiler is not heating the hot water side properly, it sometimes works (flares up) and after a short while approx 30 seconds goes out but pilot light stays lit.
I also have noticed a slight pressue drop which turns out to be the pressure relief valve, that is to be replaced asap but I don't think that has anything to do with the hot water problem as I check the pressure before running the water or heating.
Thanks for any help.