Two different makes were used (Grundfos and Bosch) and only the pump head was replaced by the plumber. Is this the one I need then?I am confused , I wondered when you said you had tried a different pump this boiler should have a Myson CP 53 pump only, no other make should be used I have seen Ideal multi-heads being used but wouldnt recommend it but they will fit
http://www.plumbclick.co.uk/circulating-pump-cp53.html
For this price I am happy to give it a try.