Thanks to MSER landkmum @ MSE for this.
It seems to let you order any dvd and get cheapest free,incl pre orders for Cars etc.
But the terms and conditions are out of date,and no mention of not including pre orders,box sets....
So basically you can get Cars dvd and Little mermaid for £17.99
It seems to give different discounts at checkout,so add to basket to see what best discount you can get.