also check out my post here
http://www.howtomendit.com/answers.php?id=164444
LTGF
August 2008
turn the mains water off to your house is the simplest way, or tie the ball cock up that is in the tank than drain it. some installation have a isolation valve on the cold water feed pipe to the header tank(loft tank/not cylinder) draining is the easy part, filling it back up usually causes promblems. See how you go on, I'll book mark this page so if you need any more info, post a reply o this page.
LTGF
August 2008
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.