Error from ffmeg - negative cts

[Chris Marriott]

I'm running the latest Windows installation of "get_iplayer" on Windows 7. 
Every program I've tried to download today has given errors while running 
ffmeg (2.1.4). First:

Non-monotonous DTS in output stream  0:0; previous : 640, current : 640; 
changing to 641. This may result in incorrect timestamps in the output file.

And then on each processing line:

negative ctsm previous timestamps might be wrong.

Anyone know what might be causing this? The resulting MP4 files seem to be 
playing OK. The error has only started happening today.

Thanks,

Chris