I had the same issue, becoming more and more frequent since I had not used it for a few months - I rang Argos' Acoustic Solutions helpline (0845 640 0800) and they offered to refund it in Argos vouchers if I brought it back in - So I assume that it is a VERY common problem. I have no receipt (as it is over 2 years old) and no proof of purchase at all, however they are aware that they are the only supplier and were happy for me to return it. Without proof of how much I paid I may have to accept the lowest price they sold it for (less some depreciation) but I'm happy with that. If you have a receipt or proof of purchase (bank statement/credit card statement) then you'll be laughing.
Mark C
March 2010