How can I fix my built in shower?

I have a new build flat and the shower looks like it is built into the wall, that is, the shower head protrudes from the wall and so does the shower valve.

The problem I have is that the water is not as hot as it used to be, it used to run very hot and now it is warm at best. The pressure is the same as always, its just a lot cooler.

How do I go about getting into the internals of the shower? The valve has 2 screws on the front of it, but other than that its all behind the tiles.

Any advice would be appreciated!