First off
Does the lockshield valve look similar to these?
http://www.wickes.co....421550/?source=123_75
If you remove the plastic cover, normally a push fit or occasionally a small screw. Quite a tight fit sometimes.
Underneath is the valve body. You should be able to see a valve gland nut. Carefully tighten this and it will probably stop the dribble (not too much or you will be unable to close/open the valve). Be aware that they are usually made of soft metal, do not use excessive force. If the dribble persists then renewal of the valve should be considered.
Let us know if you've still got problems and one the more plumber orientated ABer's will join the thread at some time.