The usual faults on normal ballcock or float valve designs are usually.
1. the valve or its washer is scaled up or in some other way knackered. the
ballcaock or float has a hole in it which makes floating a challenge!
In the first instance establish if you have a loft tank fed or a mains fed
system and buy a whole new valve assembly with the right sized hole, i.e.
big hole loft tank little hole mains fed. Also do not even consider those
with most of their parts made of plastic, they are rubbish and trying to
make the pipe join leak proof has driven many a DIY er to drink.
If its the float, If its not in too bad a nick and you can see the leak
site. let it drain or evaporate and mend the hole with something like epoxy
resin and then refit.
Otherwise you need a spare and this may or may not still be available
depending on the design. I also hate plastic bits joining this with the
valve as they have just snapped for no reason other than its crap.

Brian

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This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Jim.GM4DHJ ..." wrote in message
...

"Murmansk" wrote in message
...
I've been asked to sort out a toilet cistern that keeps on filling and so
overflowing.

Photo of offending item he

https://www.dropbox.com/s/vtfogxvkne...29.04.jpg?dl=0

Is it just a case of removing the circular blue bit and replacing a
washer or diaphragm or something inside there?

Thanks


I would just replace the whole thing ......