To be completely honest the best thing to do is to put it out of it's misery there and then and save the extra stress and expense of taking it to a vet/wildlife rehabilitator. It's perhaps just as sad that the Sparrowhawk lost it's meal needlessly because of a cat - the bird will have to expend well needed energy to hunt again. (Oh I may get slated for saying that
- sorry to be controversial but it's a valid point)