This script work with the executable of ffmpeg : https://github.com/FFmpeg/FFmpeg
It permit convert all ASS or SSA subtiles in MKV files to SRT(UTF-8).
Simply put the script and the ffmpeg exe on the directory which contain the mkv file with ASS subs and execute it.
New .MKV files will be created with SRT subtitles by default : [DEFAULT-NAME]-SRT.mkv