Sime super 80 installed about 1996/7. Regularly serviced but recent problem. Boiler stopped working so called Corgi engineer. he came out and, after diagnosis, fitted new fan. Since fitting the fan the boiler will not stay lit when replacing the combustion chamber cover. Its as if its being starved of air. I can get the boiler going and replace the combustion chamber cover loosley but then when putting the sealed chamber cover it goes off again. I'm waiting for engineer to get a reply from Sime and I have mailed them but thought that the advice here looked consistently helpful. can anyone help?