What offer have they made?
You are covered by the Consumer Rights Act which replaces the old Sale of Goods Act.
However, it is fair to say that it really doesn't change things all that much. You are out of time to reject outright as S22 CRA 2015 gives you 30 days to do so. You do still have the right to demand a repair or a replacement which should be done within a reasonable time.
In terms of which remedy you can demand it depends. They will almost certainly offer a replacement. They could try to repair though. If they do then you cannot demand a replacement if that remedy is disproportionate compared to the option of repair.
Can I clarify anything for you?