Assuming the glass is "etched", as opposed to a laminate being placed on top, you should be able to use nitromors paint remover. Remember to cover any other paintwork first.
2 weeks ago I have a New Potterton Gold Combi HE 28 boiler installed. It is great for heating the rads, very quick to reach temperature and always works. The hot water output however is intermittant with no obvious reason. A potterton engineer visited and could find no fault and it was ok...