In a new housing estate it is relatively normal (and I would say it is even good practice) for the scheme map not to be available at contract stage. My normal procedure when selling housing estates is as follows:
1. For contracts, we use the estate layout plan as submitted for planning.
2. When foundations are in and boundary walls built (or at least when the work is substantially advanced to the point that it is obvious where the walls will be) we get the site surveyed and the scheme map prepared.
The advantage of doing things this way is that the scheme map is prepared by reference to what is actually there on the ground. In other words, the physical "as constructed" reality determines boundaries and the scheme map is prepared to reflect this. This approach gives rise to fewer problems in the long run.
The alternative is to have the scheme map prepared at the outset and then ensure that everything is built in exact conformity with this. This is less practical: it can often happen that sites will be slightly realigned in the construction phase.
Of course, the scheme map must be made available before the sale is completed. It is not acceptable to close a transaction on the basis that a scheme map will be produced later (I have done it in the past, but was never very happy doing it). However, I would not be overly worried about its non availability at contract stage, esp. if the foundations of the house are already in and the location of the future boundaries is clearly discernible on the ground.