Having discovered that while everything was working quite well for MPEG content in 'film mode', like the feature film on a DVD, I hadn't really tried things with stuff in interlaced mode. Things like the 'making of' programmes on DVDs tend to have been made for television, and contain frames made up of two fields that may have movement between them.
The end result was a horrible juddering, where the fields were sometimes being played in reverse order. Objects that should have moved across the screen smoothly, were actually taking two steps forward and one step back.
That set me playing with the whole double-buffering/anti-tearing thing again. I'm still deep in the middle of trying to get things back to a working state.