baxi brazlia wall mounted gas room heater flame keeps blowing out?

My wall mounted baxi brazilia room heater is causing a problem, the flame which I can see through the small inspection window keeps being blown out.
The gas burns inside a cast iron box with an outlet to the flue through the wall. The box is covered by a decorative cover, which also acts a protection as the cast iron box gets hot when working and heat is convected upwards through a chrome plated grill at the top of the box.
I had a problem with the heater some time ago and tightened up the fixing screws that hold it to the wall. To prevent air leaking in to the burner chamber from the flue which goes through the wall into the outside world. I have done this again and I wonder if there may be a problem with the heat resistant seal around the back of the cast iron burner chamber, so air is being dragged in some how. I think air leakage is the problem. it is currently working on the lowest gas setting but not well. Any ideas?