I found this handy guide courtesy of google. I haven't done this myself though, but I'm pretty sure my Dad did so in the past:
Repairing & replacing cistern float valves
A Float Valve Diaphram
A faulty float/ball valve is more often than not the cause of the majority of problems associated with cisterns and storage tanks. Older valves use a washer to stop the flow of water, whereas modern versions use a diaphram instead which is less susceptable to scale deposits.
The float valve allows water to enter the cistern, as the water level rises so does the hollow float/ball, at a predefined point the float reaches it's highest level and closes the valve which shuts of the water supply.
If your float valve is old, the best advice is to replace it with a modern diaphram valve. These are quite inexpensive and readily available from a plumbers merchants.
Installing a new diaphram valve
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Isolate the cistern in the plumbing system, or if there is no isolating valve turn off the mains water supply
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Drain a toilet cistern by flushing the toilet
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For a water storage tank, lower the water level by running a cold bathroom tap until the water has dropped at least by 6 inches
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Disconnect the wall inlet pipe (or rising main in the loft) from the valve, and move a few inches to one side
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Using two spanners, disconnect the entire float valve assembly from the cistern
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Remove the float/ball from the end of the old valve and connect to the new one
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Take one of the fixing nuts off the threaded shaft of the new valve assembly
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Push the assembly through hole in the side of the cistern, and screw on the fixing nut to the protruding threaded shaft
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Secure the float valve assembly in place using two spanners, and reconnect the inlet pipe/rising main
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Turn on the water supply and watch closely for leaks as the cistern fills
Replacing a diaphram valve
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Isolate the cistern in the plumbing system, or if there is no isolating valve turn off the mains water supply
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Unscrew the large retaining cap on the valve, located inside the cistern at the end of the float arm
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Depending on the particular model the retaining nut will be infront of or behind the valve
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If the retaining nut is at the front then you will see the diaphram assembly immediately behind the cap
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The the retaining nut is at the rear then slide out the cartridge assembly to locate the diaphram behind it
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Before installing a new diaphram and assembling the valve, clean all the parts in cold water to remove any scale
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Reassemble the valve and reconnect the water supply
http://www.simplifydiy.com/plumbing-and-heating/cistern-valves
