The amount due at each assessment date is determined with reference to Core Clause 50.2, and Option B Clause 11.2(28).
At each assessment date the Price of Work Done to Date is the total of :
1. The quantity of work completed for each bill item multiplied by the rate.
2. A proportion of each lump sum which is the proportion of work completed for that lump sum.
So the amount due is entirely dependant on the structure of the Bill of Quantities.
Assuming there is no item for materials on site in the Bill of Quantities then I don't believe that there is any amount due for "materials on site".