Billing dispute

I bought items online from TFG. When proceeding to checkout i could not use my 25% discount code as the website was having technical difficulties (dropping off, loading errors, login issues etc) i ended up on the screen which diplayed a notification that the order was placed successful. Which i couldn't edit so i may use my discount code. When i called the customer contact center i explained the situation to the lady. The lady said i must cancel my current order and place a new one. This was not helpful as the item is now sold out in my size. I would appreciate if i could get my 25% off as it was not my fault that their website is faulty and now i must pay full price on an item whereby i have a discount code which i would like to redeem.