" if it has been clearly established, as it has in this case, that a planning consent was improperly and invalidly granted, then it should, in principle, be declared to be void. It is not appropriate that, other things being equal, the council should be required to pay compensation as a condition of achieving that result. The answer would be different if the planning consent was one which should in any event have properly been granted or where at least it appears that that might be the case. Similarly, the position would be different if there had been a material change of position on the part of an affected party on the faith of the consent being valid. Each case would depend upon its own facts and an evaluation of the relevant factors overall."