Fumes from gas stove in sitting room

Hi all
A few months ago my parents replaced the real fire in the sitting room with a gas fire. They say that some registered gas technician couldn't sign off on it because there wasn't a good enough draw up the chimney. Strange as we never had any problem with the real fire (not that I remember from growing up).

So they ditched the gas fire, and got a free standing gas stove instead. It is installed correctly, however there is a smell in the room when it is on. Most people only notice the smell, but my father cannot sit in the room for more than 10 minutes before his eyes and throat start to irritate him. As far as I know, the pipe to extract the fumes goes all the way up the chimney. Would it be possible for the fumes to come back down the chimney? Or does it mean the stove is leaking? Or is it just a smell that will go away eventually (although they have been trying it for a good while).

So now they don't know what to do. I could try to install it in my house, as I have a two-storey newer house the draw might be better. They'll probably have to get rid of the stove, but I'd like them to keep it as there is more heat out of it compared to the normal fire.
Any help or suggestions would be greatly appreciated! Thanks!