NEC does not provide for provisional sums. The Works Information should be clear as to what is required so that it can be programmed and priced.
If the Employer is not sure what is required then he should set out clear assumptions in the Works Information that can be programmed and priced and should the assumptions change then the CE process kicks in.
There is provision under X14 for advanced payments.
You do indeed have a problem. As has already been stated NEC does not provide for provisional sums and as such there are no provisions dealing with the expenditure of these sums. If the works which they relate to are not covered in the Works Information then you are not obliged to do them. Clause 55.1 in main Option B clearly states that information in the Bill of Quantities is not Works Information or Site Information. You will therefore need a Project Manager’s Instruction (clause 14.3) before carrying out any works related to these provisional sums which will then be a compensation event under clause 60.1(1) attracting any associated cost and time. You will then be paid for the work when it is completed.