Thanks everyone for the help... we now have water running through the
central heating again!

We tried flushing mains through the drain outlet, which indeed did leak
from the body (Its my mums house and she did the usual fretting to aid
the situation)! This didnt seem to fill the up stairs rads up so we
tried the same technique trhough the header tank, which after a few
burts did dislodge the sludge enough to get the cleaner that we'd added
running through.

With any luck this should clear our blocked system and we wont have to
flush rads!

Cheers again everyone, I'm off for a shower!

ct