It sounds like a duff cat to me Andy, I don't know how these manufacturers and suppliers for that matter get away with selling this junk. I have had problems with replacement cats myself and now only fit Arvin metalcats which I have found ok thus far. The only problem is they are quite pricey, but hopefully you only have to do the job once. I have just diagnosed a Fiesta with a faulty cat and the customer toddled off to a fast fit to save money, he then comes back to me because it fails again, I retest it and it's the new cat not working. He gets it replaced but it still won't pass. I eventually convince him to put the metalcat on. It only needed a warming up not even a road test and the co% was zero.
Sorry for long post but it really bugs me that they can get away with supplying goods which aren't fit for the purpose, they then come up with the excuse "you must have a fault with the car". How much time have you wasted on this job Andy, and if your customer comes back to you next MOT with a failed cat will he still be your customer after that.