If it is to provide advice and input, but not do any of the design and documents, then keep it simple: NEC4 Professional Services Short Contract (PSSC), probably on a time charge basis.
Beyond that, it depends what the 'beyond' is. If it to deliver a few deliverables in one go and which the Subcontractor can go of on do, then again the PSSC, but perhaps on an activity schedule lump sum basis. 'Beyond' that where it interacts with other parties deliverables and there are multiple deliverables, then at some point the risk/complexity matrix will start to err into the use of the full Profession Services Subcontract document.
Beyond that, it depends what the 'beyond' is. If it to deliver a few deliverables in one go and which the Subcontractor can go of on do, then again the PSSC, but perhaps on an activity schedule lump sum basis. 'Beyond' that where it interacts with other parties deliverables and there are multiple deliverables, then at some point the risk/complexity matrix will start to err into the use of the full Profession Services Subcontract document.