We have just fixed ours, basically the diaphragm deteriorated, you can buy a new one and fit it yourself (I'd def recommend another person because we had to lift/tilt the cistern). Also a new 'doughnut' is a very good idea if your cistern sits on top of the toilet pan. It's difficult to get it to sit on the sweet spot that it used to and is stops a leak.
I used this thread
http://www.howtomendit.com/answers.php?id=154734
and made a new diagphragm from some fairly flimsy plastic. We tried a stiffer plastic and it didn't allow enough suction.
whether your diagphram is round or recangular I don't think it makes a difference.
You can buy a whole new syphon mechanism which is the plastic thing that is in the middle of the cistern except we bought one that was too large, hence how we happened on the extreme DIY route.
Best of luck!
Austin, May 2010
