NEC ECC: Under Option B, can i claim 100% payment for materials off site if i provide the invoices to the PQS / Client?

No. Under option B you get paid for completed items on your B of Q at the unit rate that you have identified that is tied into the contract. Unless there is a single B of Q item for “procurement/payment of X materials” (which I doubt) then there is no mechanism for this to be paid as a stand alone item.

Thanks Glenn, much appreciated. Can I claim a 100% payment for materials on site if I provide invoices?

Still no - if this is not a separate item on your B of Q…