Chap – I had an Argon 18 frame that came with a carbon seat post. The seat post would never stay put and with a set of verniers on it, it became clear that the diameter varied along the length of the post. They sent me a new one out pretty quickly and the shop (lifecycleUK) lent me a much nicer Cyfac one until it turned up.
In this instance I’d say its the shop that’s to blame – more than likely they haven’t contacted Argon as in a busy workshop, coming up to a particularly busy time of year they’re more interested in punting out new bikes than dealing with something that they’ll make little if no money on.
If you don’t go down the Planet X route, I know a bloke who does excellent carbon repairs – you’ve got my email so drop me a line.