Selgas
Your problem is certainly caused by one or two factors those being :
1. A poorly sealed flue system or
2. The incorrect flue terminal having been fitted to suit the environment in which it is operating.
Personally I beleive you will find that when you get your gasfitter to check he will find that the inlet or cold tube is not sealed and allows the incomming air to mix with the exhausted air thus causing an imbalance within the flue sections which in turn will cause the heater to baffle and go out.
Hope this helps but get a qualified person to effect repairs – leaking gas products can kill!!! 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
