It looks that there is not much you can do. It is unbelievably brazen unfairness. I suppose the only consolation is that if your kid gets into grammar you will save yourself thousands and thousands and in the euphoria these Â£700 will be quickly forgotten.
My kid was about to go to a private school last year and I gave a deposit more than triple yours. The deadline for doing so was 4 days before the allocation results (well after they had granted me 15 days extension already!). After my child secured a place at the grammar I wrote a nice letter to them and asked them for the money back. They gave them back to me almost immediatelly, but asked for my permission to quote me on the nice things I said about the school in my letter (and I meant them by the way)!. So you never have anything to loose by simply asking (nicely
), once you have celebrated. In any case when I got the cheque I gave more than half of it to a charity to spread the cheer more widely.