Most practitioners will be familiar with R. (Nash) v Barnet LBC  EWCA Civ 1004. In that case, the Court of Appeal decided that the S.3 duty to consult was concerned with questions of policy and approach and not specific operational matters such as the identity of the outsourcing partner or the terms of the particular contract. The complaint could not be about an alleged failure to consult about a particular contractor or the terms of a particular contract; it was about the alleged failure to consult on the proposal to outsource in principle.
More recently, in R. (Peters) v Haringey LBC  EWHC 192 (Admin), the court said that S.3 requires local authorities to consult about high-level decisions, policies or approaches. In this case they should have consulted before deciding to seek tenders for a feasibility study or at least before approving the business case. Parliament could not have intended that consultation about policy and approach should occur after the local authority and its prospective partners had expended time and effort pursuing practical implementation.
So the point where S.3 consultation obligations bite is at the initial policy/formative stage, not the detail decision making and implementation later.