Central heating not working - help needed

I have a problem with my heating system which uses a Potterton Netaheat. It is a pumped system, with header tank, Grundfos pump and a motorised three way valve for hot water / heating / both. The system stopped working very suddenly when the boiler cut out. It became apparent that the boiler was overheating and the stat was quite rightly cutting in. I suspected that water was not circulating in the system, and checked the pump, which had been noisy for some time. On draining the system and checking the pump, it was found to be well gunged up and so I replaced it with an identical model. No change, although the pump started up, the boiler quickly overheated and cut out. I have drained the system, flushed it and tried again, but no good. I have re-filled, bled, drained and re-filled again, re- flushed......... no good. I did notice that the motorised 3 way valve firstly makes no noise when the system is fired up, either on heating or hot water settings. Also, I noticed that when the system was not on, the casing of the electric 3 way valve was quite warn to touch. Could this be the problem? Any other suggestions? I really think I have eliminated sludge and/or airlocks.