Ideal Standard Jetflow / Jetflo type diaphragm washer for your toilet cistern syphon - measures approx 4-3/4" x 3-1/2" ( 122mm x 89mm).
If your toilet requires several pressings on the handle to make it flush properly (or at all), then the chances are that this large washer in the syphon is damaged. They can crack, disintegrate, fold over and any one of these faults can stop the toilet flush mechanism from working properly.
Also, you should consider the following before completing your order :
When you remove the cistern from the pan you will probably find a damaged rubber washer, rather like a squashed, deformed doughnut. It is very hard to re-seal the joint between the cistern and pan re-using this squashed item. We strongly recommend you replace this washer at the same time as doing your syphon washer repair job.
Click here for a link to the page with doughnut washers