There is differences in the specification and the drawings, so which takes precedence?
No there is no hierarchy between documents within the Scope. When either party notices such an ambiguity or inconsistency between contract documents, they should notify the other of that fact. The Project Manager needs to then issue an instruction to correct the ambiguity or inconsistency (17.1).
This will then be a compensation event under 60.1(1) as there is an instruction to change the Scope, which will then be assessed in accordance with 63.10. This clause states that it will be assessed in favour of the party that did not create the ambiguity or inconsistency. That means in this case if the Client provided the documents in the Scope, the Contractor is deemed to have allowed for the lesser/cheaper of the two items. If they instruct the more expensive of the two documents, the cost and time effects of the difference between the two is what the Contractor can include within their CE quotation.
Word of warning to Contractors - check these clauses have not been amended in anyway with Z clauses, as sometimes clients can change the liability of these clauses making their mistakes the Contractor liability at tender stage!