I work for Debenhams so know the answer to this!!
For items reduced by 50 % or more, we offer exchanges only. However, for faulty items the law is different, and if an item is not fit for the purpose, they will give you a full refund within a 'reasonable' timeframe - usually 6 months for clothes.
They will be able to trace your transaction as you paid with a debit card - #I would ask them to do this if they query the length of time you've had it.
Your difficulty will be proving that the item was damaged when you bought it, and also, proving that you've only worn it once in the 3 months you've had it, and therefore it is not down to reasonable wear and tear.
My advice, (from experience and purely personal, not Debs official policy, before I get into trouble!!), is not to go in with all guns blazing (it's tempting to dig our heels in with people who are rude to us.....)Go to the Coast department and explain what has happened, how disappointed you were to buy such a reputable brand and find it did not perform blah blah, and if you have no luck there, ask to see the Debenhams Sales Manager for Womenswear. Coast is a concession - shop within a shop, and a seperate company with their own staff.
Another thing, you must to back to a store that stocks Coast - not every store does.
On a brighter note the sale starts this week, so if you settle for a gift card, you can always pick up some bargains for your lo on the gorgeous kidswear dept!!!