Filling a fireplace rebate

[Gazzamuffin]

I have a cast iron fireplace insert and limestone surround that needs fitting. Everything fits in the to cavity no problem, but there is a 3 inch rebate built into the surround. That means when the insert is sitting flush with the inside of the surround legs there is a 3 inch gap between the wall and the back of the insert.

If the gap was quite a bit less I know I could just fill it with fire cement, but 3 inches seems like a bit too much.

Does anyone have the best way in which I can create a seal between the insert and the wall?

Many thanks

 

[bathjobby]

I think there may be building regs in this area, but even so, your probably better off posting this in the Building forum before you get someone in here suggesting you pack it with newspapers.