the problem is not with the Flushing unit but with the float valve. You might find that if you take it out carefully, and descale it that it will work again. If it is a "quiet" unit, then it will have a complicated air-block system, which although clever and quiet is probably impossible to repair once it has started giving problems. I suggest replacing it. Homebase do replacement float valves for both rear fill and side fill cisterns. Make sure you work out what you need. They cost about £10.50. I replaced mine yesterday and it works perfectly.
Jeremy Meyer
April 2008