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Usual (and easiest) way to do it is to put a cap on the top of the chimney, block up the fireplace with thermolite blocks and plaster over the top, remembering to leave a vent to allow any moisture that penetrates into the chimney to get out.
What I have described above us about 100 times easier than ramming that amount of shit down a chimney so someone must have been really desperate to save in skips.
What's the proper way to block up a chimney? This is what I found. Seems less than optimal.
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