Replacing a toilet siphon valve, or as it is also called, a fill valve, is a simple and easy DIY job. The siphon valve regulates the flow of water into your toilet tank and determines the water height within the tank as well. Most siphon valves are universal, so you should be able to find one at any local hardware store to replace your defective one, and as of May 11, 2010, they cost under £5.
Close your toilet's shutoff valve. It should be located directly behind the toilet bowl.
Unscrew the supply line connected to the toilet, running from the shutoff valve. Use a pair of channel locks to get a proper grip to loosen the nut.
Unscrew the plastic nut on the bottom side of the toilet tank. This nut locks the toilet's siphon valve in place.
Remove your toilet's siphon valve.
Insert the new siphon valve through the hole of the toilet tank where the previous supply valve sat.
Place the siphon valve gasket between the toilet tank and the base of the valve.
Set the small plastic hose coming from the top of the siphon valve into the opening on the top of the flush valve. The flush valve allows water to flow into the toilet tank when flushed.
Place the washer over the threaded end of the siphon valve on the underside of the toilet tank and screw on the plastic locking nut onto this threaded end.
Screw the supply line onto the threaded end where you just thread on the plastic locking nut.
Open the shutoff valve, which will fill the tank with water.
Flush your toilet to make sure everything is working properly.