Sounds like diaphrams gone replace diaphram or whole syphon. Go to your plumbing store and ask them if this can be removed without dismantling the cistern from the toilet. That's the trouble with these fancy units they don't allow for access.
Remove top and take a digi picture of syphone they might be able to tell you if it come apart without the need to dismantle. other than that it's demolish the unit.
Nobby
November 2008