I have a Twyfords syphon which has a similar problem to similar question.
I have endeavoured to remove this to either replace, or take to plumbers merchant.
How do you remove the old one?
Do you have to disassemble the plastic cover beneath the cistern, to get to the syphon.
The cistern is about 12 years old

mike, September 2008

I assume it a close coupled toilet.

According to www.twyfordbathroom.com it requires seperation of the close coupled cistern from toilet.
Turn off the water to cistern disconnect the water feed pipe. if it hasn't an inline check valve in the water feed pipe time to put one in while off.

Undo the bolts under cistern that hold the cistern to pan [you will probably need wd40].
Remove the screws holding the cistern to the wall if any [under the lid at the back]
once all free its ready to be lifted off the pan. Undo syphon securing nut remove syphon. Run to plumbers get another and a new doughnut washer for fitting between pan and bowl, don't reuse the old one. Reassmeble in revers. Oh! you best get two new cistern to pan securing bolts at the same time.
So EXTRAS
NEW DOUGHNUT WASHER
MAY BE NEW DOUGHNUT METAL PLATE IF THERE IS ONE.
NEW CISTERN TO PAN SECURING BOLTS
INLINE CHECK VALVE IF REQUIRED SAVE TURNING OFF THE WHOLE SYSTEM.
AND OF COURSE THE NEW SYPHON

BOB, September 2008