The Professional Service Subcontract would be most suitable as the timescales would fit in with the main Engineering & Construction Contract (ECC).
Neither the NEC4 PSC or PSS has an X22, but it would be relatively easy to take the ECC's X22 and adjust IF the Contractor is willing and the project circumstances fit. As a general comment on X22, I do not think it has sufficient termination provisions if a Client decides not to proceed to Stage 2, especially if they wish to use another contractor.
Alternatively, the Contractor - who is not subject to EU rules for procurement - could let one PSS contract for Stage 1 and once Stage 2 is confirmed, let another PSS contract to the same Consultant for Stage 2.
Neither the NEC4 PSC or PSS has an X22, but it would be relatively easy to take the ECC's X22 and adjust IF the Contractor is willing and the project circumstances fit. As a general comment on X22, I do not think it has sufficient termination provisions if a Client decides not to proceed to Stage 2, especially if they wish to use another contractor.
Alternatively, the Contractor - who is not subject to EU rules for procurement - could let one PSS contract for Stage 1 and once Stage 2 is confirmed, let another PSS contract to the same Consultant for Stage 2.