We have just ordered a set of these \link{http://www.hippo-the-watersaver.co.uk/index.html\Hippo the Water Savers). You can get a set of 3 for £6.99 including VAT, P&P. You can pay immediately by Paypal too.
Apparantly, "each Hippo will save on average 3 litres of water (appoximately 33% of the cistern volume) every time a toilet is flushed. Independent research has shown Hippo will provide a 10-15% saving on total water use."
Been recommended to me, so thought I would pass on the information :)