Home Builders Federation (HBF)has published a voluntary code, Customer Service Code of Conduct, which states: The home builder will cooperate with duly qualified professional advisers appointed by the client to resolve disputes, if they arise.
If the builder has difficulties to allow you access, but having both parties interest in the final deal, your lawyer will be able to arrange access amicably. This has not been a major problem lately as developers are used to dealing with us, knowing that it is a legal right of the buyer.