you dont say if your boiler is a combi or not. if it is.
1. does the boiler go on and off regularly with hot tap running? if so. plate heatexchanger getting blocked. will need replacing and system flushing out.
2. run hot tap,(with heating off) and check if radators are heating up,if so,diverter valve faulty. hope this helps
corgi 110
January 2010