NEC ECC: Use of provisional sums as a Z clause?

‘We are currently tendering for a circa £2m project under NEC4 Option B in which the following Z Clause has been inserted - Is it reasonable/workable?

Insert new identified and defined terms as follows:
11.2 (34) The Bill of Quantities includes Defined and Undefined Provisional Sums in accordance with the definition in the RICS New Rules of Measurement (NRM2). A Provisional Sum is a sum provided for work whether or not identified as being for defined or undefined work. Provisional Sums included in the Bill of Quantities are included in the total of the prices.
Insert new Clause 60.8 as follows: The Project Manager issues instructions in regard to the expenditure of Provisional Sums included in the Bill of Quantities. Provisional Sums are deducted in the computation of the assessment of the final amount due and the change to the Prices assessed in accordance with Clause 63 are added. A Project Managers Instruction for expenditure against a Provisional Sum is in accordance with the definition in NRM2.’

The project programme is very tight and delay damages have been stated as £1,000 per day. Under the above clause and NRM2 definition it would appear that tenderers are expected to make due allowance in their programming, planning & pricing of preliminaries. There are 27 defined provisional sums included in the tender BQ totalling £484K (only a few of the 27 meet the NRM requirements for a defined provisional sum)
We are aware that the NEC recommends that provisional sums should not be used in their contracts and realistic programming for these 27 items is not possible with the information provided. Do you think the above clause is reasonable/workable under the spirit of the NEC ? Can we annotate any allowances within our initial programme as ‘provisional’ and adjust accordingly once the design is available for these works?

Frankly no I don’t like to see provisional sums in NEC contracts as the machinery of the contract is really not set up to deal with it properly. If approx. 25% of the work is so ill defined that it needs a provisional sum then this probably shouldn’t be under an Option B contract at all.

I suspect the above will just lead to problems. At best the contract will be ignored and at worst it will end in dispute.