I had a similar problem when I added another couple of radiators - when I
looked in the header tank it had a load of red 'algae' type stuff which
had
beeen crusted around the sides and bottom of the tank, but had been
stirred
up by the emptying and refilling.
This had blocked the downward feed to the system, giving the symptoms you
describe.

Fixed as follows:

(1) Tie up ballcock to prevent more water coming in
(2) Drain header tank into the bath using garden hose as syphon - the
syphon
was primed from an upstairs tap (mains cold water) because I didn't fancy
sucking. This filled the bath with nasty red gunk but cleared out the
tank.
You can untie the ballcock for a bit to help the flushing through.
(3) Removed the bottom feed from the header tank and fixed the end of the
garden hose to it secured with a jubilee clip.
(4) Connected other end of the hose pipe to mains cold water and slowly
turned on the tap. This pressuirised the system, and cleared the blockage.
Fortunately I have more than one hosepipe so I could flush the system
through by opening the drain cock downstairs and draining through another
hose pipe. Hopefully this removed most of the red gunk.
(5) Reconnect the pipe to the bottom of the header tank and fill as
normal.

HTH
Dave R

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
!
Thanks again,
Steve