I'm sure Sally feels a lot better having delivered her pearl of wisdom. I'm sure you agree it speaks volumes about her character.
Right, You could try bending the lever arm down (if its brass) to ensure the shut off level is below the overflow. If its plastic there should be screw adjustment to make the ball cock shut off the water at a lower level. However it is more likely that the ball cock rubber washer is hardened or perished allowing water to seep past the closed valve
All ball cocks can be repaired/overhauled but to be honest it's not worth it for someone who doesn't know how to or doesn't have the spare washers and seals.
Go to B&Q and buy a new ball cock for your header tank and you will also need 1 X 15mm tap connector washer. This washer forms the seal between the water feed pipe and the ball cock. Total cost less than a tenner
Now mention to a friend who is a reasonable DIYer that you have got all the parts to replace the ball cock but you can't undo the the fixing nuts because they are to tight or your wrench doesn't fit. Voila!
|er i think plumbers do that sort of work if you can't.|