Have you considered getting a pair altered by a dressmaker, or the best bet is to try the school shops online as not many high street shops stock navy. We have the same trouble getting a knee length skirt in navy, but can only find at the school uniform stockists and the older traditional makes.
I got some in the end from the school shop, £16. I did find them in places like bhs and next but found the fit was not good for her shape. I will try M&S next time I go to a large town that has one. Thanks all.