sounds like your gas valve has gone but you can not change this yourself, check with Vokera for a fixed price repair, as the valve is quite expensive and needs to be set up for your boiler, a decent RGI can compete but a fixed price repair from Vokera would be a good idea, SSE are doing a thing just now for £150 and that would probably be cheaper than you buying the valve but some of their engineers are doubtfull to say the least