LIZS - just curious - the C850 web page still says 329.99 so did they have a discount code or say "in store only" (or could it have only been yesterday that price was valid) ?
Also, when I looked at the mobile page OP linked to it had a special offer (well, seems to have it for more than just that one) saying "Save �20 when you buy Windows 7 with your new Windows PC today. Offer ends 18 January 2013." (am assuming � is pound symbol on this iMac).
That implies that they are now selling Windows 7 to "downgrade" some machines. Perhaps not intended for this one if it had been running Windows 7 but even so, it is a little puzzling that they installed Windows 8 and for those wanting WIndows 7 would have to pay extra (less the 20 quid discount)... It would surely break Trading Standards on being wrongly described, and I would be asking Argos to provide a copy of Windows 7, and not a replacement laptop.
Having tried both, then when using mouse and keyboard (rather than a tablet/mobile for which Win 8 was designed) I found it clumsy and less easy. Admittedly I've not even used Windows 7 that much (gave my BiL my laptop in 2010 a few weeks after buying it, because of networking problems) but going to such a graphical only menu system is the opposite of progress in my view.
Windows 7 is more widely available, and is clearly written on the Argos description. If a family computer is going to be usable, then I hope for OP's sake that school / work also have Windows 8, else it will mean some things are quite different and awkward on Win 8.
I had an instance with Tesco Outlet on Ebay, selling me a Toshiba 40BV705 TV when the listing was actually for the 40BV702 I didn't bother to complain (they could send it for just 3 quid but it would probably have cost me 20 to send back the TV and while similar spec, was not identical, but still good enough as 4 HDMI ports all work, fortunately).