halstead ace no hot water?

my ace boiler is not so ace. the hot water is playing up if you turn it on then down to a trickle it was staying hot but now it wont even do that and it keeps blowing off through the pressure release valve.I have changed the dhw diverter switch diaphram as it was perished but the problem remains. please help as i need a BATH.I may be wrong but I think it is the diverter itself.