The correct treatment depends on who the original supplier of the items being claimed is "contracting" with. Perhaps in many of these cases there won't be a formal contract as such, if so consider who that original supplier would regard as their customer, and whether the goods concerned would at any stage "belong" to the subcontractor.
If the contract is really direct between that supplier and you, the subcontractor is acting as your "agent", paying on your behalf. Even if the subcontractor is VAT registered, they can't claim the VAT charged by the original supplier and shouldn't add VAT to the amount they paid when they claim from you - it's just a disbursement in their books. You can claim the VAT element using the original supplier's invoice.
But if the contract is really between the original supplier and the subcontractor, the treatment depends on whether the subcontractor is VAT registered:
- If they are VAT registered, they claim the VAT on the original supplier's invoice. They should then bill you for the net amount paid to the original supplier, plus VAT, which you can reclaim using the subcontractor's invoice.
- If they aren't VAT registered, they can't reclaim the VAT on the original supplier's invoice. Their claim to you will be for the VAT inclusive cost, but they won't need to add VAT again on top of this. However, you won't be able to reclaim any VAT on your payment to the subcontractor, as you won't have a VAT invoice in your name.
So if the subcontractors are VAT registered, it shouldn't matter as the ultimate cost is the same either way. But if they aren't VAT registered, it's better for you to have the "contracts" direct between the original supplier and you - even if the subcontractors then collect goods and pay the supplier on your behalf.