Rob - I currently have the same problem so I rang the manufacturers, in my case Ideal Standard. They couldn't have been more helpful. Push either of the buttons down to flush the toilet and hold it down. Then get a small flat head screwdriver and use it to gently prise out the one that isn't pushed down. This comes out quite easily and then you can get the other to do the same. You'll then find a central plastic screw and you need to undo this to open the cistern. Note though that the problem is highly likely to be caused by blistering of some parts and they will need replacing I believe. The brilliant woman at Ideal Standard is sending me the replacement parts and they'll talk me through the installation when they arrive
Pat Langford
November 2008