no it doesn't cost to call british gas out for a leak or suspected leak, ie. smell of gas. i would most definitely call them right now, as your senses could be getting used to smelling the gas and thats why you think its not there all the time. be safe rather than sorry, espically as it doesnt cost you. look up the number right now. and as for the landlords certificate of safety, id ask to see the certificate for yourself and dont be fobbed off by the agents. its your lives at risk here if anything was wrong.