bad smell in my living room that seems to be coming from the gas pipes

You seemed to be a little confused. Gas pipes do not 'run to my radiators' only heated water does. The smell/odour of gas is very distinctive and certainly cannot be confused with the apparent foul odour you can smell. As natural gas in its basic state is odourless, a chemical is then added during processing to give it that unique smell. But better to be safe then sorry report the matter to National Grid by telephoning 0800 111 999 its a free service they will attend if necessary even if it is just to eliminate a potential gas escape. If no gas escape has been detected then I would suggest you call a qualified plumber to investigate further!