You can read the full decision here: uksc-2016-0188-judgment (1).
Particularly interesting are paragraphs 60 and 62 :
60. Finally, with regard to Sales LJ’s concerns about the burden on members, it is important to recognise that the debate is not about the necessity for a planning authority to make its decision on rational grounds, but about when it is required to disclose the reasons for those decisions, going beyond the documentation that already exists as part of the decision-making process. Members are of course entitled to depart from their officers’ recommendation for good reasons, but their reasons for doing so need to be capable of articulation, and open to public scrutiny. There is nothing novel or unduly burdensome about this. The Lawyers in Local Government Model Council Planning Code and Protocol (2013 update) gives the following useful advice, under the heading “Decision-making”:
“Do make sure that if you are proposing, seconding or supporting a decision contrary to officer recommendations or the development plan that you clearly identify and understand the planning reasons leading to this conclusion / decision. These reasons must be given prior to the vote and be recorded. Be aware that you may have to justify the resulting decision by giving evidence in the event of any challenge.” (their emphasis)
62. The Model Council Planning Code and Protocol, already referred to (para 60 above) contains under the same heading the following advice:
“Do come to your decision only after due consideration of all of the information reasonably required upon which to base a decision. If you feel there is insufficient time to digest new information or that there is simply insufficient information before you, request that further information. If necessary, defer or refuse.”
This passage not only offers sound practical advice. It also reflects the important legal principle that a decision-maker must not only ask himself the right question, but “take reasonable steps to acquaint himself with the relevant information to enable him to answer it correctly” (Secretary of State for Education and Science v Tameside Metropolitan Borough Council  AC 1014, 1065B). That obligation, which applies to a planning committee as much as to the Secretary of State, includes the need to allow the time reasonably necessary, not only to obtain the relevant information, but also to understand and take it properly into account.