A professional heating engineer came round today, in fact the very chap who fitted the system back in 1999.
Using a very methodical approach he found that a T junction just above the pump was the problem. It had clogged solid with black gunge. He chopped this out and replaced it.
He fired up the system and its all working fine, he even put in the system cleaner I was originally trying to put in.
In 4 weeks I will drain it all out and put in an inhibitor and maybe even a descaler and make sure this is replaced every couple of years. Its worth it for peace of mind. It was B**** freezing these past couple of nights, so lesson learned.
Thanks for all the advice and to everyone who took the trouble to reply.
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