This isn't going to be easy, Bramley. Non-standard fitting by the looks of it.
There are plenty of replacements, but they tend to be round fittings. I did see one oval shaped, but nothing like yours ............
http://toiletspares.co.uk/spare_buttons_28.html#
This is a real pain sometimes, with all the nice, stylish (non standard) suites available. Bathstore is one of the worst offenders.
Are there any markings at all on the cistern, or the syphon mechanism?