Extracting mpeg4 file from pcap capture

Hi, I have a multicast Mpeg4 part II video stream Having joined the stream on my PC, I can view the incoming packets in wireshark, and save the capture as a pcap file

With the appropriate filters, I can then filter this capture to show me only the UDP video traffic, then decode this traffic as RTP

If I then add a further filter such that the first packet is the start of an i-frame, then I can save the payload as an m4v file (vi the telephony>stream_analysis menu) and play it in VLC media player

However, this will only play in some players (like VLC). In addition, if I change the extension from m4v to avi, VLC will not play the file.

My guess is that this is because VLC is taking the m4v file extension as a hint as to how to decode the file, and without it fails to play. It presumably needs to do this because the file itself is missing suitable mpeg headers describing the stream...but I'm guessing here. Can someone give any pointers as to whether I am doing anything obciously wrong, and how I might go about rewriting the mepg headers if this is indeed the problem?