How to identify material or labour at PO level for CIS and then assign withholding tax

We are in the process of implementing P2P modules for one of our UK clients and on of their requirements if to implement the CIS (Construction Industry Scheme).

Since this is for the constructions industry we are going to heavily use the complex purchasing functionality.

The challenge we are facing now is how to identify the material and labour separately on the purchase order, since withhilding tax is only applicable for labour only. However if a PO has both labour and material,how do we create invoices automatically with ITEM lines including withholding tax ( for labour ) and ITEM lines exclusing withholding tax( for material ) on the same invoice