Fix my front door lock

I have a simple Yale lock on the first of two front doors to my flat. The lock mechanism itself still works perfectly well, but the lock box (if that's what it's called) that is attached to the door has become loose and wobbly. It feels like if I leave it, the lock will end up just falling off. I have tried tightening the screws that you can see in the picture below, but tightening them does not seem to make the lock box any more attached to the door. So I need someone to come and work out how to secure the lock to the door firmly again. I do not need someone to change the locks or fix the lock mechanism, it is just a case of making sure that it is properly attached to the door. In your quote please detail any experience doing this kind of thing before. I will most likely be looking for someone to come round and do this on Wednesday 5th Sept sometime in the morning. Cheers! (Photos are attached below)