Problem With Gas Fire

WE got a new gas fire into a new house a few years ago. We only use the room and the fire on certain occassions. After the fire has been on for a while, we notice there is a 'sooty' smell in the room but no real smell of gas. Has anyone else had the same problem? The installing plumber has spoken about a smoke patch test. He also asked re chimney and how clean it is but I understand the 'anti-bird' contraption on the top was put on straight away after chimney construction. Of note we have seen some kind of device on another house in the same development with a H type thing on the top and ? if the 'pull' up the chimney was a problem.

Sorry I can't help with query on fire but just wonder if as a precaution you have installed a for safety.

Did you line the flue? If not, the smoke/hot air/vapour from your gas fire may be cooling before it gets out the top of your chimney, causing the air to sink back down into your room, bringing the smell from the inside of your flue with it.

Do gas fire chimneys need to be cleaned? And if so how often?
We have one in our house (we don't use it that often) but bits of black soot seem to get on the carpet and are impossible to get out properly.
Can anyone advise?