A manufacturer is free to offer whatever period of guarantee he chooses, or no guarantee at all. Sony products carry a 1 year guarantee:
http://www.sony.co.uk/article/id/1159199102480 /page/1
A manufacturer's guarantee is completely separate from your statutory rights. Go back to the
retailer (not Sony) and state that you're confident that the failure of the power adapter must be due to an 'inherent fault' (e.g. poor quality components). Challenge him to suggest any likely alternative reason. Then demand that he repairs or (more likely) replaces the adapter free of charge (which the retailer is obliged to do, in respect of a defect caused by an inherent fault, for
6 years after purchase).
See my recent post, here, for further clarification:
http://www.theanswerbank.co.uk/Law/Question671 460.html
Chris