what do i fill this gap with

[torchy663]

Hi

inside the fireplace, there's a gap between the mantle and the chimney of about 2" wide and 30" long

when the previous house owner fitted a mantle and marble back for a gas fire he didn't fill this gap in

B Gas have now told me its unsafe and disconnected he fire as gases can escape behind the mantle and into the living room

question have is what can i fill this gap with, foam, mineral wool, some insulation material, what?

its really awkward to get to, I have to lie on my back with my head in the fireplace so i can look up and do the job

[welsh brickie]

you can either use a fireproof foam,or fill the gap with expanding metal and sand,lime,cement.