Broken syphon flush?

The flush on my 'Ideal Standard' toilet has suddenly broken, or at least I can now only get it to work after concerted effort whereas before it never had issue.

The mechanism seems to be that the handle tries to force water up an n-shaped tube which then draws the rest via syphon, but flushing now doesn't seem to force sufficient water over the crest of the 'n'. There's no apparent broken part, but then the key mechanism is inside the sealed plastic 'n' that I can't access...

Any ideas would be well appreciated!