Hi, thanks very much for your question. My name's XXXXX XXXXX I'm going to assist you with it.
Your principle rights in this matter arise under the the Sale of Goods Act 1979. Under this legislation the suite that you purchased is required to be of "satisfactory quality" and "fit for purpose". Broadly an item is "fit for purpose" if it is fit for the everyday purpose it was reasonably intended to be used for. Failure to meet these standards will provide a right for you to claim against the retailer for breach of contract.
In essence, the nature of these rights will be determined by how much time has passed since you received the goods and when the fault arose. If faults had been obvious when you received the suite, you would have had up to around 3 weeks to reject the goods and obtain a full refund. If the fault arises further down the line (as in your case) you will no longer have the right to reject but you still have the right to have the faulty items replaced or repaired. You can ask for a replacement but the retailer will ordinarily choose the least expensive option, which he is usually entitiled to do.
Whichever he chooses it must be done within a reasonable time and, if he does not take action, you can then either retain the goods and make a claim for a reduction in the purchase price or claim recission of the contract, which means returning the goods and obtaining a refund (less an amount for usage of the goods to date).
If the fault arises within the first 6 months, the retailer has the burden of proving that the goods complied with the standards under the Sale of Goods Act. For instance, he would need to prove that the tear was caused by an external factor outside the everyday normal use of the item. More than 6 months and you need to show that the item was not fit for purpose or of reasonable quality. This is, as in your case, usually achieved by instructing an expert. The fact that your expert appears to agree that the leather is of poor quality is very helpful to you.
On that basis there should be little difficulty with demanding a replacement or repair. The problem is, as I say, you may not easily be able to convince the retailer that a replacement is required. However, if he does insist on a repair and (as the expert predicts) the repair is of poor quality, you can continue to complain until the matter is satisfactorily resolved or threaten recission (as described above).
If you want to contact the retailer, pending his receipt of the report, you can find a good template letter here: http://www.which.co.uk/consumer-rights/action/letter-to-ask-for-a-faulty-item-to-be-repaired-or-replaced-/
Within the letter, you can choose replacement (rather than a repair) and make clear that a repair is inappropriate as the poor quality of the leather renders the suite unfit for its everyday purpose. Make your expectations and intentions clear to the retailer that you will not be satisfied with a repair to the current tear, as the leather is deteriorating elsewhere. Bearing in mind the expert analysis, he may be inclined to offer a replacement but there are, of course, no guarantees.
Can I clarify anything for you?