Could be the diaphragm on your syphon unit has gone. If you take a look at this picture: http://www.flickr.com/photos/26046989@N05/2445809417/ it has a picture into the top of a toilet that was failing to flush which had been getting worse at flushing for a while. The problem was a tear in a plastic sheet which lifts the water when you push the handle. The solution is to buy and fit a whole new "syphon assembly". The syphon assembly is basically the whole central plastic section shown in the picture with the two pipe tower that has 'Derwent MacDee' written on it. You can buy the replacement assembly from a good hardware store such as Leyland SDM in London or from a plumbers merchant. As they don't tend to advertise on the internet very well I suggest that you search for 'plumbers merchant' at 192.com which turned up a large number of them.
Here is how to replace the syphon assembly:
1) Isolate the water supply which is filling the tank. In our case there was a brass pipe running up to where the water enters the tank. In the brass pipe was a silver compression joint with a flat screw head on the side. Turn the screw head 90 degrees to the left to shut the valve. Now if you push the ballcock down no water comes in so you have isolated the water supply.
2) Empty the toilet tank. We didn't have a plastic pipe to syphon it so I unscrewed the ballcock and bailed it out with a large mug. Took a while. When the mug couldnt get anymore out I used a small towel to soak the rest. This is important as you get the assembly out by unscrewing it from below.
3) Get below the tank. The fat pipe were the water goes into the toilet is actually secured with two large nuts. One large nut holds the pipe in place, the other olds the syphon assembly straight within the tank. You will need a large wrench to get the screws undone. This cost a tenner from a hardware store.
4) With the nuts undone the syphon assembly is now detatched from the tank. You now need to use a pair of pliers (or two) to straighten the wire holding the handle lever to the rod in the top of the syphon assembly. With that done you can lift the assembly out of the tank.
Turn over the assembly. Inside the box section of it is a rod, spring and plastic diaphragm. Our diaphragm was torn. That had been making it harder to flush intil it wouldn't any more. Effectively what that diaphram does is lift up the water until it goes up one tower in the syphon and falls down the main pipe. If it is torn you wont lift the water and it does not flush.
Now I read that you can buy a new diaphram. However the whole assembly was less than 8 pounds at a hardware store. So it seems safer just to buy a whole knew one which is sure to work. To assemble it just go through the steps above in reverse.
simon massey
April 2008