if you can definately see or even smell the fumes coming back into the house then this is classified as an immediately dangerous appliance and you should stop using it and call a corgi-registered gas engineer in for a safety check right away.
corgi have the best of intentions with gas safety but its not a guarantee as there can still be incompetent gas engineers out there, we come across appliances all the time that have not been fitted correctly and are potentially dangerous.
some engineers pride themselves on dong a great job and some are just i it for the money i'm afraid.
if your i the south birmingham area, solihull, redditch or bromsgrove give me a shout.