I'm sorry they misinformed you to go to Citizens Advice, and telling you to ' have known better',  was wrong too.

If this was Private seller, who has a No Returns policy on their listing,  that seller can refuse returns for the reason change of mind,  or does not fit.

If purchased from a Business seller,  they have to receive the item back for a full refund,  but you the buyer needs to pay for that fully tracked return postage.

Those are the rules, and  not a case for Citizens Advice!

@k-winters