WOT - I hit them with - 'I bought new stock, you sent me B stock, I would be happy to keep it with an appropriate discount'. So far as I can see that is a sensible option that costs everyone involved less time, money and hassle.
Warren - in a way you are right, had I opted for a full refund straight away then the whole thing would have been done and dusted. But, rather than all the wrangling over how much discount I might/might not get, the thing that has annoyed me is the supercilious way I have been dealt with.
Anyway, my email to the boss prompted contact from the head of Customer Care and I have now decided to take a full refund and move on.