You sound like you know what you're doing - are you using a removable filling loop? If you are it may well have a one way valve in it to stop heating system water flowing back into the mains supply (I know this shouldn't happen because mains pressure is higher than your heating system pressure - but they have to play safe) Try putting the filling loop back on the other way around and see if water flows properly - I think it might!
If you DON'T have a removable filling loop then it sounds as if mains water is unable to access your system - look for a valve/isolator that may have been shut off and will need opening to allow flow.
Good luck.
Richard
January 2009
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