New fireplace insert question

[chromey]

I'm installing a new cast iron insert into and existing fireplace opening in a Victorian house. I want to use the same wooden fire surround and mantle which are already there.

After removing the wooden surround I can either...
a. fit the iron insert against the chimney wall brickwork. This would leave a gap (60mm approx) between the inside leg of the wooden surround and the front of the insert, which I guess I could make good with wood shims and paint, or...
b. I could bring the iron insert forward away from the existing brick work so its against inside leg of the wooden surround. This would then leave the 60mm gap between the existing chimney masonry and the iron fire insert. If I do this do I need to bridge the gap with brickwork/blockwork or is their an easier/better solution?

I ask because when I had an installer fit an insert in a similar fireplace he went for the second option and made up the gap with expanded metal lath (Expamet I think?) which was wrapped in what seemed like mineral wool insulation. But I can't quite remember how he did it...

Any tips much appreciated.

Incidentally, I assume I dont need to be a HETAS installer to do this work?

Thanks.