Hello,
Under S.14 of SOGA all items must 1) be as described, 2) be fit for purpose & 3) last a reasonable length of time.
Obviously your case fits in here.
Under SOGA it is YOUR decision of what you want done, be it a replacement, repair or refund. And as it is under 6 months old, there is the assumption that the fault existed at time of purchase.
You can demand a replacement if you wish, as your statutory rights cannot be excluded due to 'store policies'. I shall say however that SOGA states that both parties must be reasonable and allow a repair if it is not economically viable to replace.
If the sofa was say 7 months old, it is best practice to accept a repair, and if a fault develops again, you would expect a partial refund (to account for the length of time you had it).
You may have a fight on your hands against a sales person, as most people aren't educated in the law, and they've probably been told that a repair is a must before replacements - you need to speak to the manager directly.
If however the manager still insists the repair, simply ask for the head office address, and send them a letter with your complaint, keep a copy of the letter and send it by a recorded means (for proof of delivery).
If you need any more assistance, don't hesitate to ask
Adam