Thank you Dave and Stuart for your helpful comments.
The off-screen rotation won’t be a problem, as I will be isolating the inner face features with an elliptical aperture mask and due to the relatively small presentation of the videos, any scaling I may do shouldn’t cause a big problem.
I was having problems with one particular video (trying to track at or near the eyes) but had the blinking issues as you mentioned. I think I’m just going to have to go with the “best” results for him, rather than perfection. However, I tried tracking the corners of the glasses of another guy and it worked a treat, really excellent results despite quite a lot of head movement.
Shame they don’t all have glasses.
Thanks a lot for the help.