NickH

Like Theyomper said modern loos overflow into the pan so it could be that, look to see if the cistern is overfilling and then overflowing into a tube in the centre of the flush mechanism. If not, then another thing is that, instead of a syphon they use whats called a "dump" valve, appropriate really, which is basically a rubber washer sitting snug on top of a round hole, holding the water in the cistern, its right at the bottom of the flush mechanism. When you push the flush button the washer lifts and lets the water go, now if the washer doesn't sit back down properly then the valve will let water by. You could have some muck in there or the valve might not be dropping down properly. Also is this a close coupled loo. On concealed cisterns with a plastic flush pipe you can get a similar problem if the flush pipe is pushed to far up into the cistern, it pushs the dump valve open and lets water by.