Reducing size of brick fireplace opening to fit inset fire

queserasera_2

Took chimney breast back to brickwork as all walls needs re-plastering. Wanting to fit an inset living flame gas fire (with marble hearth and backboard) but the existing fire opening is too big. Is it possible to reduce the opening in the brick fireplace with a heat retardant board or anything similar underneath the marble back board? 
I have tried contacting a few local fireplace company's to do a pre-installation check but no one seems interested unless your buying their products. 

FreeBear

You can use something like cement board to reduce the size of the hole. Although, I'd use a light weight cement block (breeze block) and perhaps back fill any unused space with vermiculite.

If you are not doing the grunt work yourself, who ever is fitting your gas fire would be best placed to do the in filling.