I would of thought that the normal procedure is for the contractor who is building the extension to engage their own engineer to carry out the survey before any work commences.
Both the contractors client & the neighbour should be giving a copy of the report.
This puts the onus back on to the contractor who is engaged to carry out the works.
You will need to have your contractor verify, by way of a written document that the engineer he is using is suitably qualified, charactered etc.
The engineer could be a consultants office working on behalf of the contractor.