For some reason the Guliverkli version of the filter that Yaobing linked here didn't work for me, while the Celtic Druid version did. So, I got it to work in MC this way:
1. Download the
packed binary file (build by "Celtic Druid")
2.
Extract the filter file (flvsplitter.ax) from the archive
3. Copy the file to your "windows/system32" directory
4. From the Start menu choose "Run" and enter: regsvr32 flvsplitter.ax
[5. (added by JimH) From Matt's post below -- configure ffdshow (Start Menu > FFDShow > Video Decoder configuration) and tell it to use libavcodec for the FLV1 format ]
Hope this might help others although I think it's coming from my computer that gives me daily signs that it's about time for me to reinstall windows...