Take the cistern off the toilet and undo the big nut on the bottom to release the syphon mechanism. This will probably mean disconnecting both the water in and overflow pipes. The syphon has a worn out diaphragm which takes about a minute to change once you have the mechanism off. Either buy a new one (£1.50 @ B&Q) or cut one out of some plastic. Easy job.
Peter
November 2005