I used to be a shop manager and have had similar situations myself. but i had to no the law on these things. so here comes my rant.
By LAW you have 7 days to return anything weather it be electrical or not (they wish to test it due to company policy) if they dont have to refund you your money (due to some shit company policy) but they should offer you a credit note at least, for the full amount and for anything in the store. i would go back and say you wanna see the manager. if he/she is a cock/cow say you'll go trading standards. because thats against the sale of goods act. you can print stuff of about this from there web site. hope this is some help fella