Someone will come along shortly to provide first hand advice, but what I took from a thread which followed my asking the same question reference a tv was that yep, you can return it for a full refund even after opening / testing, provided you didnt pick it up in person!
Consumer right? Can I open mail order TV to see if too big for lounge?
Distance selling regs – absolute right to return for any or no reason within 7 days.
Open it – check if its OK – if its not pack it back up and return it at your expense
DSR doesn’t have any requirement to return unused (or in a resellable condition) and would overrule any company’s ts and cs, but only for mail order.
Course, that won’t stop some shops trying to ignore the rules.