It depends a little on the wording of the procurement procedure within the Works Information. If it was an express provision to follow a certain process then possibly, but it is unlikely to be so prescriptive. The very nature of option D is that the Contractor is incentivised to minimize their Defined Cost spend in order to lower their final costs and increase their "gainshare" at the end of the project. What would be in it for the Contractor to pay more for something that they needed to? They will have paid out that money and decreased their gainshare (or increased their painshare) so it should not be in their interest to do so.
The other key thing is we all realise now that cheapest is not always best. I do agree however that there should have been a justification from the Contractor as to why they did not go with the cheaper supplier.
I certainly do not sanction audits at the end of projects which I have seen where on reflection costs are disallowed as it was felt they could have been lower in the market place (not that you are suggesting that here).
The other key thing is we all realise now that cheapest is not always best. I do agree however that there should have been a justification from the Contractor as to why they did not go with the cheaper supplier.
I certainly do not sanction audits at the end of projects which I have seen where on reflection costs are disallowed as it was felt they could have been lower in the market place (not that you are suggesting that here).