Got a video of my squat form for people to critique
my 2nd set of 120kg and I was feeling it since doing squats on Monday as well.
http://www.mattmccann.co.uk/squat_form.MOV
Great depth, but you may find you lose some of it after adjusting slightly.
1) This is the most important point; you are bending your lower back. In fact your back is never in a strong position, resulting in it collapsing fairly early on. The lower back rounding is probably also caused by tight hips/glutes/hams as well.
2) The pad is a real problem and will do you more harm than good. In your case it's exaggerating the problem you have with your back, by having the bar even higher up your spine you are increasing the forces on your back. This means that when your spine is out of position and becomes loaded your back muscles have to work very hard to stabilise. A partial fix here is to make sure you push your chest out throughout the lift.
Also, you mention shoulder flexibility restricting your bar position. Try this:
http://www.mobilitywod.com/2011/04/episode-211365-the-simple-five-way-shoulder.html
Other than that, it's actually fine - although it is important that you correct your back.
The only other thing I'll mention is that you are basically doing a high bar squat. Your form isn't terrible for this, you stay relatively upright (apart from the back problems I mentioned) and your knees looks to be out. If anything you should aim to exaggerate this.
For a low bar squat, obviously the bar needs to be lower, but also you might need a wider stance and you'd have to be even more careful with your back.
...I'll have to get the tub out later
Going to the gym tonight so will have some wonderful aches in the car tomorrow no doubt.
). 
