The Sale of Goods Act implies certain terms about the goods being of satisfactory quality and corresponding to the description. Given that under at least one interpretation of the measurements (ie. theirs) the mattress the mattress does correspond to the description then you would be, in my view, on rather shaky ground if you issued a claim for the refund at Court.
If there was a defect with the bed which resulted in it not being of a satisfactory quality then you could possibly claim a replacement or a refund depending on the seriousness of the breach, but on the facts it does not really seem to be a quality issue.
In my view I would do a bit more sabre-rattling, tell him you are aware that the Sale of Goods Act implies certain terms about quality and description regardless of the fact that it was sold at clearance. I personally would aim for a replacement mattress from them rather than a refund of the whole. State that you do not wish to issue a claim but will do (as a bluff) if necessary. Try to reason with him and see if you can get a replacement mattress out of him.
If it does not work you can seek specific advice from a litigation solicitor for them to specifically appraise your claim but it sounds as if you would be starting on a bit of a thin ground and you could get hit for their legal fees (as a store they would certainly defend with a lawyer) in the event that you lose. These fees could over £2, 000.00 for a start.
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