“In the practical application of section 18A it will obviously be necessary for the decision-maker to consider the development plan, identify any provisions in it which are relevant to the question before him and make a proper interpretation of them. His decision will be open to challenge if he fails to have regard to a policy in the development plan which is relevant to the application or fails properly to interpret it. He will also have to consider whether the development proposed in the application before him does or does not accord with the development plan. There may be some points in the plan which support the proposal but there may be some considerations pointing in the opposite direction. He will require to assess all of these and then decide whether in light of the whole plan the proposal does or does not accord with it. He will also have to identify all the other material considerations which are relevant to the application and to which he should have regard. He will then have to note which of them support the application and which of them do not, and he will have to assess the weight to be given to all of these considerations. He will have to decide whether there are considerations of such weight as to indicate that the development plan should not be accorded the priority which the statute has given to it. And having weighed those considerations and determined these matters he will require to form his opinion on the disposal of the application ….”