Rachel, obviously I can't say how this occurred, but if you are sure it did not happen in your care, your contract is with the retailer, so go back and telll them that under the Sale of Goods Act, you require a replacement or a refund (refunds are due under 6 months I think it is).
It is their responsibility, under Consumer Law,to correct your situation, not Sony - although Sony have probably done you a favour, by saying it's been there "some time".
You must tell the shop it was obviously not "fit for purpose" due to the problem, when they sold it to you, as you have never got the thing wet/damp, etc...which it sounds very much the situation.
They have to prove the opposite, and it sounds very much like Sony's phrase in their report - do get a copy if you can - supports you. Under consumer legislation, that phrase would almost certainly be considered more than the 6 weeks it's been in your care. The shop would need to show it wasn't, which Sony would appear to disagree with.
Look up the Small Claims Court procedure, tell the retailer that you consider Sony's supported your opinion, and tell them you'll happily see them in the Small Claims Court if they don't simply replace it, or refund you.
Have a quick look at the Sale of Goods Act, it is there to help you claim your rights - and even a quick look will almostr certainly mean you know more about your rights than the retailer !
All too often they haven't a clue - or, sadly, occassionally simply lie, to get out of their responsibility.
Hope this helps.
