Dave,
You are a lifesaver ! If you live anywhere near Milton Keynes, I'll buy
you a beer, or several !
I drained the system down again and, lo and behold, the header tank wasn't
draining, so I disconnected the outlet pipe and jubilee'd a hosepipe to it
as per your instructions. Turning on the hose led to a sudden bout of
gurgling, then water started spurting out of the open valve of a
downstairs
radiator (which I took to be a good sign!).
The system refilled perfectly, better in fact than it has ever done in the
4
years that I have been living here (and I've been through the
draining/refilling routine more times than I care to remember). The smart
new rad. works well and we won't have to have cold showers tomorrow
morning
!

So did you use the mains just to unblock, or have you filled the whole
system via the hose pipe? (just thinking it wouldnt have any corrosion
inhibitor or anything in if you did that?)