If a contractor employs a sub-contractor without the consent of the architect, is the contractor obliged to pay the sub-contractor for the work they've completed?

Yes. The contractor is legally obliged to obtain consent prior to sub-contracting. This is one of the central tenets of the contract between employer and contractor. Should a contractor engage in such an agreement without consent, the sub-contract remains valid and legally binding, and the contractor is still legally bound to honour it.