Currys Discount Code Honoured and Then Removed

I bought an integrated Fridge/Freezer (F/F) from Curry's for £349. When it was delivered we realised it was the wrong door split...ordered a 70/30 and it should have been a 50/50. So Curry's took it back there and then. Phoned Curry's and arranged a swap for exactly the same F/F but a 50/50 door version. That was cheaper at £299, and said they'd refund the difference of £50. Only received £10, and when I queried it, they said they will not honour their own online discount code of £40 I used. It's not as if a new order number was generated, Curry's amended the existing order with the new F/F but removed the discount code already applied to that same order. If I had bought the correct F/F originally, I would have received that discount. So beware if you make an order error, and have to arrange for a swap, you'll loose any discounts if the promo is no longer running then.