If you are not allowed in by the developers to take measurements, where are you getting the blue line (actual wall location) from?
If something is not built in accordance with planning permission, you could have two issues:
1. Your bank might not approve your mortgage
2. You may have problems when you go to sell in the future.
Under the building control regulations, the builder will have to certify that what he has built is in accordance with the design (i.e. what's on the ground will have to reflect the site layout plan & other drawings as submitted to the planning authority).