Thanks for the advice Oskar - I will have a look for what you describe when i get the courage to take the end off!
I think though from fiddling with the buttons and peering through the grille that its actually the levers which are sticking rather than the buttons themselves in the holes. There seems to be some lateral movement of some levers if I move the buttons side to side, and the levers of the RHS C and G row top buttons get too close I think and interfere with each other.
Does anyone know if I can fix levers that are loose like that?
Many Thanks
Geoff