Just in case anybody actually takes any notice - for future reference; This subject came up today with a client and contractor. As I was explaining it, I realised that I hadn't mentioned planning conditions, which are very important to this. You can only rely on a technical commencement if all planning conditions are satisfied within the period of the planning approval. In other words, if the approval states that work must commence within three years, and there is a planning condition to submit details for written approval, those details must be submitted and approved in writing before the three years expires.