My first bit won't initially seem relevant but stick with it and I hope that you'll see why it still is really:
If you'd discovered a fault with the sofa after it had been in your possession for more than 30 days you'd have the right to a repair or replacement (but not to a refund). You could nominate which of those two options you'd prefer but the seller would still be entitled to choose the other one if it was cheaper or easier. (i.e. you could say "I want a replacement" but the seller could lawfully say "Sorry, I'm going to repair it").
However you noticed the fault within 30 days of receiving the sofa, so that means that you're entitled to a
full refund. If you choose not to go down that route though you're then back in the same situation as if you'd not noticed the fault until much later on. (i.e. even though you want a replacement, the seller is entitled to only provide a repair).
So, as the law stands, you can't insist on a replacement. However all is not lost because you can use the threat of demanding a full refund as ammunition in your battle to get a replacement. (i.e. "The law says I'm entitled to a full refund and that's what I'm going to demand unless you provide a replacement, rather than a repair").
https://www.which.co.uk/consumer-rights/regulation/consumer-rights-act