Try bending the arm that holds the ball, down so it tends to go further under the water when it rises.
Not a plumber but just an idea.
.
POM SAYS ...
Push the ballcock down, screw the screw in and make sure the locking ring is secure.
SUGAR SAYS ...
Hope this makes sense because it's all ballcocks to me :shock:
It sounds like maybe the flow cut-off washer might not be fitted correctly. If the ball arm is made of brass (or another soft metal) you might be able to put a downward kink in it to lower the ball. That sometimes works but you do risk snapping the arm so it should only be a last resort.
If I was nearer I'd offer to pop round with my tools! :-)
In edit: Bluddy heck... where'd all those replies come from! :shock:
Possible problems:
Could be leaking from the nut behind the ball valve within the cistern. If it's this, you'll need to reseal it with a strip of PTFE tape.
OR
Could be something jammed in the seal within the ball valve (grit or similar ... well, actually Pom said 'crud' but I didn't want to say that) - if it's this then it'll need to be fully dismantled and cleaned. Then the screw will have to be adjusted. To adjust the screw porperly, the ball valve must be pushed down to release the pressure from the screw.
(Rather you than me ... Sugar)
Damn, Sugar/Pom, I was hoping to avoid taking the bloody thing apart again. There was crud in the old one (I took it apart to see how it worked) which explained why it always took ages to fill. But this was new - but it was a cheapy so maybe I shouldn't be surprised.
There was a red grommet sort of thing in the pack but nowhere to shove it, no idea what that was for.
That red grommet (rubber seal) should go between the ball valve and where the pipe comes into the cistern. You may need to replace the old one as it's obviously leaking.
Excessive pressure can sometimes cause these seals to pass. I assume you isolated the water supply to change the ball-cock, did you turn it on the same amount when you had finished? Have you got increased pressure at your taps (more than before)? Just easing the pressure back slightly may solve the problem.
Good luck.
I hope u have fixed the problem by now.
If the valve isnt shutting off and its a new one my instinct would be to make sure the pipe that comes up, that the waters comes in thru, is secured against the side of the cistern. If it isnt the pipe bends and u never get the water shut off.
The water that comes into your toilet cistern will come from either the mains or a tank (gravity feed) somewhere on the house, usually the attic.
Inside the inlet pipe gubbins, where it connects to the adjusting screw, is a changeable cone shaped plastic piece. This needs to be either for mains or gravity feed.
The gravity cone allows more water flow. If you have mains pressure and a gravity cone, the water may well find its way around the cone and continue to flow.
So try checking for that.
Some good advice i can see but:
as duncan says it could be a problem of too much pressure or maybe not enough
Have you changed it like for like, and does it have a big ball on the end or does it have a small float.
If you have a water tank in your loft, and its in the bathroom (downstairs cloak should run of mains) then its either you have the wrong pressure insert in it they are red or white, small nozzel mains, big tank fed. Or as someone else mentioned you could have some crud in it especially if you drained down the tank if you could not shut off supply.
They work on the pressure of the water coming in to seal and any grit in the way it will not seal.
If you still have problems then pm me and i can go into more detail, it would much easier to diagnose if i knew what heeating/hot water system you had.
Lets see
you have checked to see all seals are in place
ensured the water pressure is not too high
made sure the washer is the right one
Right take it apart, look at the washer. Is there a make on it it? Is it a full circle? If the circle is incomplete the other side of the seal is damaged. Check the other side anyway, it should be flat! Ensure the washer is well seated and reassemble.
It will not leak..............
turn the water on and hope!
who can advise? Is there an alternative type or do I just keep buying and fitting them until one works? 
:rotflmao: :rotflmao: :rotflmao:
