They are entitled, at this stage, to offer a repair or replacement.
As a little more help, this page of the site may help explain your rights:
Returning Faulty or Damaged Goods | Consumer Information
"If a repair is impossible or unfeasible, you must then be offered a replacement. Due to the emphasis on proportionality in this legislation, you must give the seller reasonable time to repair or replace before demanding your money back and you should be aware that any refund given may well take account of any use you have had of the goods since you took possession of them. If you do not want the seller to repair or replace, or they have told you they are unable to, you can then request they reduce the purchase price to an appropriate amount, although this does not affect your ability to take return the item if something else goes wrong"
Hope that helps.