You know what? I just formatted my memory card and put the movie file back on, and it plays fine without any picture juddering - that's from the original rip - before running through Media Converter, too.

I wonder if one of the music players I have on my Nokia was scanning the cards music files, hence the slowdown of movie playback?

I have no problem playing back converted DVD's using mediaconverter (same parameters as Martin: 400x240, 512, 64) and playing with UKMP. Just now tried it again - very smooth, excellent quality. There have been times when I began watching and there were occasional glitches. I found that when I switched the n800 to offline mode, the glitches disappeared. As I recall, this usually occurred when I was out of wireless contact so it may have been related to the n800 periodically trying to establish wireless contact.

How do you go into offline mode? Is it just a case of disconnecting from the current network you are connected to?

To enter offline mode, briefly depress the power button (right button on top of n800), then in "device mode" menu, highlight "offline mode" and click "OK". This mode turns off wireless and bluetooth radios.

I will soon be loading my music tracks back onto the memory card - as a result I expect the movie playback to start failing. Is there a way of checking what processes are running so I can determine what program is the resource hog once I have filled the card back up?

I use a status bar plug-in called "load-applet". See https://garage.maemo.org/projects/load-applet/
There are also ways to do this at a lower level with xterm but someone else will have to advise you. I know just enough about linux to be a danger to myself! By the way, the "Search" feature at the top of the page will give you the answer to just about any question you may have.