Insulate in Recess of Fireplace

[BingoBango]

What's the best way to insulate the recess of a fireplace (when I say best I mean both cost effective and safe!).

 

I'm using quite a lot of heat out of the back of the recess (when they did cavity wall insulation this bit wasn't insulated).

I've had the following suggested - is this safe and is there a preference (and if I fix it to the wall then what do I put on top of the heatproof boards - if anything?).

 

1). Skamotec 225 - Skamo Enclosure Board

2). Vermiculite fire boards

3). Calcium silicate Insulation boards - very expensive

4). Rockwool FirePro / RW5 (non-combustible mineral wool)

 

e.g. could I use the Rockwool FirePro and then cover it with a Vermiculite Fire board and paint over the top?

 

I'm more than happy to reduce the size of the recess - it doesn't need to be as big as it is for us.

 

Thanks