Acqualisa thermostatic shower problem with red button that will not depress?

The red button has got stuck, it will not depress which means we can only have cold showers as the button prevents the lever from being turned towards the hot water. I believe this is a child safety button to prevent anyone being scalded with an overly hot shower.
Does anyone know if this is a common problem, can it be easily fixed? To resolve the problem does is the outer casing need to be taken off, and if so how do you flip off the 'acqualisa' disc?