Sorry to keep bombarding you with videos but as my back is feeling okay I decided to do some squats. I was hoping you guys could have a look and tell me if I'm doing anything that could hurt my back (this was my last set at 90kg). I notice when I'm getting to the bottom my weight seems to be coming off my heels? This seems to be a problem all round for me, haha. Maybe it's a flexibility thing?
Spread your legs (hmmmm...) and keep your knees out.
Better ankle mobility will help, so ditch your trainers and break out the 2.5kg plates. You'll need to work on your adductor mobility (because it's going to hurt as a starting point) and your glute med strength for this, but it will pay dividends.