Preparing a chimney breast for a new gas fire

Hi All
I hope someone can help me with a building prob. We recently removed a gas-fired back boiler from the chimney breast of what is now our lounge in favour of a condensing boiler in the garage. SWAMBO now wants to put a basket fire in the old hearth most of the ones we have come across so far state they are suitable for a brick flue. What we have is a brick flue but!! for the old gas-fired boiler the flue has a stainless steel liner. The chimney breast itself is completely hollow (it is possible claustophobia permitting to virtually stand inside it). At the top left-hand corner is a small opening to a brick flue going up to the chimney. The stainless steel liner comes through this opening and at the moment simply dangles inside the breast.
I would like to leave the liner there, but realise that gasses would build up inside the hollow chimney breast. I understand there is a way to seal the empty space and leave just an exit through the liner but I am not sure how to do it. A neighbour has done something similar and this involved setting a 6mm aluminium plate in the breast with a hole to pass the liner through. It also involved removing considerable brickwork for access in the front of the chimney breast and replacing it again. I feel sure there is a simpler method.
Although we are not intending to put a fire in immediately, I am doing some other work around the chimney breast which will involve some plastering and would like to get the mess done with once and for all. SWAMBO wants to get the room decorated for Crimbo and I suspect it will be a bleak mid-winter for someone if I do not get it done.
Richard
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bes.ltd.uk has proper closure plates, tape, etc, all the stuff you might need
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Ta OldScrawn but can you give me any more details like an address and a tel. No. I cannot find anything on the net.
Ta Tricky Dicky

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