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.
One other thing has just occured to me I know you're doing a lot of the graft yourself but be careful clearing the shit out of the chimney. Given how tightly packed it is there is a very real risk of stuff getting held up and then spontaneously freeing itself to come down on your head.
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.