Well it could be argued that you may have been ordering discs for someone else, so Overclockers werent to know. And if you were buying the discs as well as the writer Im sure it wouldnt have been too difficult to check that they were compatible before pressing "submit order", as a system builder one would assume you are competent.
Im not trying to dig or anything but you cant assume they will check every order to make sure that everything in the order was cmpatible, it would cause a nightmare , as some people do order different kinds of discs at the same time (Im pretty sure anyway)
As for not refunding/replacing, I agree it is rather bad practive on there part, (although I dont agree with saying you spend £x amount of money (-it does not really add much to the argument) Also if you have spent so much surely you realise they are generally goood and not very risky to deal with) They may argue that you should have again checked the format to the drive (which is easy enough) and that they wouldnt be able to sell them on again now.
Lots of points I know, probably a bit of a muddle as well - but hey thats how my mind works!