How Can I fix my gas tumble dryer?

I have a White Knight Gas Tumble Dryer, Model BG447. It is 4 years old and has a lot of use! The problem at the moment is that it starts up, flames ignite but go out very quickly (less than one minute). What sort of problem might this be - and how can I get it fixed? British Gas wont come out to fix it and Crosslee want to charge £128!! Thanks for any help!