Most likely cause is that the diaphragm has split in the siphon unit.
Some siphon units are designed to be serviceable without removing the cistern, unfortunately most are not. If the siphon is blue leave another post.
Lift the lid of the cistern and have a look at the top of the siphon unit. If it has "Dudley Turbo 88" on the top of it you are in luck. Dudley have been wise enough to design the siphon so that it can be replaced without removing the cistern - pull out the yellow pin and the siphon assembly can be removed from the top. Either renew the flappy piece of plastic or fit a replacement siphon. Fingers crossed!