BRITONY COMBI BOILER?

HI I HAVE A BRITONY COMBI BOIILER...THE HOT WATER IS GETTING HOT FINE BUT THE RADIATORS ARE NOT...MY PLUMBER SAID THE DIVERETER VAVLVE NEEDS TO BE CHANGED....IM NOT SURE HOW MUCH ONE OF THESE THINGS COSTS AND WHERE TO GET IT FROM....ALSO WILL THIS SOLVE MY PROBLEM...OR WILL I HAVE TO KEEP CHANGING PARTS ONB THIS BOILER?????IS IT ECONOMICAL FOR ME TO JUST REPLACE THIS PART???