Phil,
Couple of notes...
Not sure if its just my monitor, but the "blacks" in the video are very hard to distinguish, so seeing exactly where your knees are is hard with dark pants and dark ground. That said...
1) You primary problem in my opinion is a significant back angle change, which I believe is caused from a) letting knees slide too far forward at the bottom of the descent, and b) shoving your knees and ass BACK to compensate rather than driving your ass UP when coming out of the hole. Because your knees and ass push back out of the hole, then to keep the bar over mid-foot requires you to make your back more horizontal, leading to more moment force on the entire back segment, which is f-ing up the concentric portion of the movement.
2) Yes your knees come in a bit as you bounce out of the hole - this is normal (though still not good - and will get better with work, which it seems you are already doing.)
3) Remember that the knees track forward a little and OUT at the beginning of the descent, then STOP tracking forward and just keep shoving OUT for the rest of the descent. The ascent is the same - where the knees keep their position (actively shoving out) after the rebound and do not move BACK until the top half of the ascent. When the first movement out of the hole, is shoving the knees back, -the ass moves back and up, the back bends over, the chest caves, and its a domino effect of problems from then on.
Matt