Cast Iron fire surround - how to seal it to the masonry to be air tight?

Hi,

I am installing a victorian cast iron surround.

There is currently a gap from the previous fireplace and the surround fits neatly into the hole. However, the masonry that the surround needs to join up to is very uneven - there is over an inch difference in places - you can see a photo of how it looks at

What is the best method to fix the cast iron surround to the uneven brickwork and how should I fill the spaces in between? I have read up a little and seen various suggestions including firerope, fibreglass or lime plaster but none actually explain how to make a wall flat and fix the cast iron to it...

Any suggestion very welcome.

Thanks

GP