Selgas
Perhaps these hints may help you work through the problem:
1. Check that the lever arm on the ballcock float has not just simply got itself stuck in the ballcock valve end – wiggle it loose and see what happens.
2. Check the ballcock float does not have a hole in it which would allow the water to get into it and cause it to stop floating and therefor stop the whole mechanisim from working.
3. Replace the ballcock washer in the valve seat where the ballcock arm fits into it.
4 If all else fails replace the ballcock valve asembly which is freeley avaialble from most Plumbing outlets.
Hope these help ya. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
