Capital allowances - timing of expenditure & stage payments

My client (Co A) built a new building between August 2007 and May 2009 on land it already owned.  The total construction cost was £4m. 

Co A occupied the building on completion.  The builder was a related party (Co B) who invoiced Co A monthly.

Co A capitalised the stage payments.

I have a full priced breakdown of the construction cost so I am able to prepare the capital allowances analysis.

My question concerns how to treat the stage payments as some of them fall into the period pre-integral features.

Do I simply treat the expenditure as incurred on practical completion or do I need to split the claim by reference to the invoices?