I haven't seen one of those for over 15 years, they were old then and been replaced.
I used to service loads when at Bg ( in the good old days) and were an absolute pain getting the fire front back on because of the gas fire supply pipe, it used to run the full length of the fire, and always got trapped in the wrong place.
I would be seriously looking at ways of Id the appliance, flue liner faults, burner any perished gaskets ect, I'm sure the customer has had their moneys worth.