thanks all for your help.
I went to see Coach Sreve Ross on Tuesday for a form check and he has fixed me up very nicely .
For the start I am using a box set at 32 cm to get the feel for what real depth should feel like .
And yes, he also caught the fact that I was locking the knees too early, basically guaranteeing that later on in the squat I would need to slide my knees forward to hit depth.