In your heading you said the couch is brand new so those amount of marks definitely aren't acceptable unless it was sold as "seconds" or damaged? If you paid full price for it I would want a lot more than a £10 refund, even if it is going to be covered in blankets. I think they're being a bit cheeky!
If you can live with the marks I would decide on a realistic figure that you'd be happy with and ask them to refund that amount, or come and collect the couch. If they refuse you could mention trading standards (as mentioned above), which should hopefully make them more accommodating!
Good luck. Let us know what happens!