Gas Fire flue issue

[torchy663]

Hi, first post for me

gas fire issue.........the previous owner fitted a mantle and marble hearth / back, however he's left a gap of a couple of inches behind the mantle back. This would allow fumes to enter behind the mantle and possibly into the room. What's the best material to fill this gap with, its approx 2 inches wide by about 30 inches long

the fire's been disconnected.

thanks

torchy