I would feel a lot more comfortable dealing with Waterstones - but the lack of a receipt means I really don't have a leg to stand on
Although the units haven't even been out a year, I can't prove that it was bought from Waterstones (though I have the original box, I'm unsure as to whether it's Waterstone's own barcodes etc on the reverse).
Perhaps the serial number could work? Sony were able to confirm proof of purchase when I gave them the date and place of purchase, so maybe Waterstones can do this if I give them the serial number of the model - certainly worth a shot!
If I were to take it back to Waterstones I would need to pay £27.50 to Sony to send my own one back, if I don't pay that then they'll "dispose of it"!!