I have a lovely pair of sandals from Fly - these www.flylondonshop.co.uk/womens/all-shoes/yidi190fly - which I am barely wearing because the strap at the back slides down as I walk. It's a problem I've had with other sandals, so it must be something to do with the way I walk, shape of feet or something. Has anyone had good results with heel grips, or any other brilliant solutions??