It's probably the washer inside the ball valve. Some are easy to match and replace, and if possible I would suggest an O ring rather than the cloth type. Others seem specifically designed to be awkward and you're better off just replacing the whole darn thing. It's not a terribly difficult job to do. Isolate the water, empty the cistern, remove the old valve from the tap connector (under cistern) by adjustable wrench followed by the back nut that resides immediately above the tap connector and fit the new one in reverse. Be careful not to cross-thread as float valves these days have plastic threads while the tap connector is almost certainly brass and be cautious about flow-rate. If it's too vigorous (it might be what knacked your old one), then either use an in-line flow restrictor or adjust an isolation valve that should be dedicated to your lav.
scarywalton
November 2007
howtomendit.com for free repair help, information and advice.
