At some stage during the life or your toilet it will definitely let you down and the problem will more than likely be with the flushing mechanism. The first thing you can take is to remove the lid from the cistern and have a good visual inspection to try and see if anything obvious stands out. Look at the lever and handle for the flush mechanism and make sure that no part of the linkage is defective. When you have checked this out, you should now come to the conclusion that the problem is more involved. It could mean that a valve washer is defective. This is called a diaphragm washer. As long as you are confident to do so, you can go ahead and take the washer out. However, in order to remove the washer you will need to isolate the water inlet to the cylinder on the toilet. Once you have done this you must flush the water out so the cylinder empties, which will let you carry on. If necessary you can use sponges towels and cloths to wipe out the cylinder. Regents Park Plumbers can supply diaphragm washers. The cisterns have something called a flush pipe, so you have to undo a large nut that holds it to the siphon. From there you need to undo the large nut that fixes the siphon to the cylinder. Once undone, you should now be able to lift the siphon out of the cylinder. Unclip the mechanical linkage of the lever arm, and if the arm of the float operated valve makes it difficult, then, you may need to take it out also. A Regents Park Plumber can fit diaphragm washers. Disconnect the hook that attaches to the top of the shaft, this pulls on the diaphragm. The housing for the diaphragm should now drop away from the bottom of the siphon so you can take out the washer. Now fit the new washer and re-assemble the unit.