“2. The supply in the course of the construction of— (a) a building designed as a dwelling .... of any services related to the construction other than the services of an architect, surveyor or any person acting as a consultant or in a supervisory capacity.” “4. The supply of building materials to a person to whom the supplier is supplying services within item 2 ... of this Group which include the incorporation of the materials into the building (or its site) in question.”