I think I've sussed the problem. The battery door seems to have a half and fully closed position. I've been closing it until I hear a 'click', but with a little further pressure, it'll 'snap' into place. I'll carry out further open/close tests to confirm if this is in fact the 'fault'.
Ah! User error! In IT we often call that PEBKAC - Problem Exists Between Keyboard And Chair.