Job sorted yesterday but not without hassle.
After turning the water off, you disconnect the cistern from the inbound water pipe and at the wall screws. Then there are two butterfly nuts on the underside of the bowl projection under the cistern. Undo those. The cistern can then be removed and the syphon can be unscrewed at the bottom.
DIY stores will only sell the complete syphon assembly when its just the diapragm thats bust. Wickes £13, B&Q £22. Wickes also sell close coupling kits at about £5, these contain the joint between the cistern/bowl and the metal plate that anchors the butterfly nuts etc.
Problems - the wire hook connecting the handle to the siphon may be different between the old & new - use your old one. The new joint between the bowl and cistern may be much thicker than the old one so the water pipe wont reach and the wall mount holes are too low. Re use the old gasket. I found both problems so had to keep pulling apart again which took yonks.
Still, never done this before, have now and its working perfectly.
Adrian, March 2009