How can I fix my Potterton Ultra2 boiler?

When my boiler first starts up, everything works perfectly. i.e it starts to heat the water and all indications are normal. After a short while there is a strong smell and flow of gas coming out of the exit flue but no heat. On checking the boiler itself the "flame on" indicator light is still showing green. Is this a terminal problem for the boiler or is it a simple (i.e. cheap) fix for Corgi man.