1. It depends on the individual. Some people will have their land registry map marked before the building commences, some won't do it until all sites are marked out on ground, some won't do it until they are under pressure from the first house completed and sold who want to register their title.
2. The land registry map is only created to show the various sites in transfer and easements to be registered. So if a new access is opened which does not affect the easements to the people in the estate then the land registry map may never be changed.
3. All purchasers will, eventually, get a copy of the map, or at least their solicitor will. Once the first transfer is registered anyone can get a copy of the map by applying to the land registry and paying a small fee.