1. Put it this way. If I was a solicitor (which I'm not) & was acting for someone buying the new house from the developer I would be extremely concerned that the easement worded as you say would not be adequate.
2. On the face of it, no-one has the right to come onto your land to do anything without your agreement. However, if the services run under your land (presumably to serve the existing neighbouring house) there must be easements given to the service companies to allow this. Copies of these should be with the deeds of your house (or on the land certificate). They may give the companies the right to access the service pipes etc. without further agreement from you - but this right may be limited or qualified. You need to look at the wording.
3. It may be you have a case to negotiate a payment to allow your neighbour to do what he wants. If the development cannot proceed without your agreement this could be a substantial part of the six figure sum. You could need specialist agent/surveyor help on this once the legal bit has been clarified beyond doubt.