No wonder you're sad. If you were using it for the purpose for which it was intended and following the instructions correctly, you have a right to a refund
/consumer rights rant
(, Thu 28 Dec 2006, 16:33, archived)
www.oft.gov.uk/Consumer/Your+Rights+When+Shopping/your+rights+buying+goods.htm
(, Thu 28 Dec 2006, 16:37, archived)
Write to their head office, explain that it went wrong for no reason, and also tell them that the employee in the shop threatened you with legal action (which he did, as he accused you of fraud).
[edit] and if you can, go back to the shop, and find out the employee's name.
(, Thu 28 Dec 2006, 16:37, archived)