Any plumbers here?

[stevestrat]

I have an "old" cold water system with a storage tank in the loft. The overflow started dribbling a good while ago so I replaced the diaphragm in the float valve and it has been fine for a couple of years but its not shutting off fully again. Replaced the diaphragm but it made no difference I was going to replace the float valve itself but its located at the rear of the tank and the inlet pipe to valve connection is all but inaccessible.

Any suggestions why a new diaphragm hasn't cured it?