SimonJ,
I have the same problem and am also wondering what rocket scientist Ideal Standard hired to design their toilets. Checked www.ultimatehandyman.co.uk and this worked:
"If it is a dual flush one, so there will be one large button and one small one you remove it by pushing and holding down the large button whilst wiggling the smaller one. The smaller one will then pull out with your fingers then remove the bigger one.
There will then be a screw that needs removing before you can remove the outer ring or cistern lid.
The buttons have a little snib on the back of them and they need to be dislocated before they can be removed.
Once you have done it you will find it much easier the next time."
bnrsmith
September 2008