I've spent a bit more time with Kinovea this evening and I think I've now pretty much got my head around the features I plan to use. I'm convinced that this freebie package will do what I want and is a far better option for me that spending a load of money on DartFish. The package has it's limitations but in all honesty I think they are not truly package imitations but the result of me doing this on the cheap with a little webcam.
I think the disengagement of the tracking "lock" I've experienced is very much cadence related. Filming at 30fps the lock remains good up to about 65rpm and then starts to fail, even with markers. I think this is because the tracking uses colour AND object shape and as the cadence rises the apparent object (marker) shape changes from round to oval as a result of the pedal speed and tracking is lost.
I'm hoping that using the PS3 Eye, if I can get it going, will allow me to film at maybe 50fps (up to the USB2 data transfer limit anyway) which might allow me to use cadences of about 70-75, we'll see. The camera was only £15 so it's worth an experiment.
So, did some initial trial recordings of myself this evening, nothing serious, just experimental, here's the outcome: