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
http://www.isite-design.co.uk/fireplace/photo1.jpg
I realise I need to make an airtight seal between the cast iron plate and the wall before fixing the wooden mantel.
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
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snipped-for-privacy@gmail.com wrote:

You don't really need "air-tight" if very-low-flow-rate will do. I'd just pack in fiberglass like I was caulking hull planks. Might need some tinting to make it inconspicuous- no biggie.
J
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