JCT: Validity of Variations issued by the employer rather than the CA

A contractor working under a JCT MW2016 contract has had numerous variations instructed either verbally or directly by the employer. The works have been carried out but the costs not agreed. The client is now citing the need for all instructions to come from the CA / Architect, as clause 3.6. They are refusing to acknowledge the variations despite the works being complete & they have taken possession of the property in question.
What grounds can be used to pursue variations issued by the employer & not the CA.
Presumably the issue of revised drawings & scope of works by themselves are a variation instruction where they show revised works.

There are a couple of ways of looking at this but they lead back to the same conclusion that the Employer cannot make changes and then hide behind a technicality. You could say, the contracting party is the Employer not the CA so while these may not be “variations” as defined they are still contractual changes suggested by one party then accepted by the other. Alternatively, you could say that for the purpose of instructing Variations the CA is only acting as Agent and an Agent cannot have more power than a Principal. There are further more complex arguments around representation and estoppel but they are probably not needed and again lead back to the same result. Finally, if you were to stick to the absolute letter the Employer had no ability and was therefore in breach in issuing Variations, a party cannot profit from its own breach and therefore the instructions would still need to be priced.

Whichever you look at it the argument being put forward to avoid payment is very unlikely to succeed, if the employer wont agree escalate to adjudication after taking appropriate specific advice.