TMART Appalling Internet

I bought a digital photo frame which took ages to dispatch despite them taking my money immediately. When it arrived it was the wrong item - in order to return it for a refund I had to pay 23 p&p - which they will only refund 15 of! I repeatedly e-mailed their customer service to re-state that it was their mistake for sending me the wrong item (in fact I think they did it deliberately and hoped they'd get away with it, as the item I wanted wasn't in stock but they didn't want t refund me) - but they say it's their terms and conditions. Would never use again - and am still to receive a penny refund.