You've not mentioned how long you've had the settee but, from the content of your post, it seems unlikely that it's a very recent purchase. Therefore you've legally 'accepted' it, meaning that you no longer have the right to a full refund.
Thereafter EU legislation requires that the retailer rectifies any problem which comes about due to an inherent fault for a period of 6 years. Such rectification can either be by making a repair or by providing a replacement. (The customer has the right to specify which of those options he'd prefer but the retailer can substitute the other if the customer's choice is not economically viable). The retailer could also offer a 'pro rata' refund but the customer is not obliged to accept it.
An 'inherent fault' is something that was wrong with the product at the time of purchase (such as the use of defective materials or sub-standard production and quality control methods). Unless there's clear evidence to the contrary (such as a knife cut where you put an axe to the settee or acid stains when you emptied a car battery over it) there's an automatic assumption that any problem arising within the first 6 months after purchase must be due to an inherent fault.
After 6 months is up, the customer must show that (based upon the balance of probabilities) the most likely cause of the problem is an inherent fault. (It's often suggested that a report from an independent inspector, paid for by the customer, would be needed for that purpose but simply challenging the retailer to offer any alternative explanation might actually suffice).
However it needs to be remembered that 'fair wear and tear' is not covered by the legislation, which accepts that (for example) it would be unreasonable to expect something like windscreen wipers to last for 6 years. A court might rule that the acceptable level of fair wear and tear on a settee bought for under £100 from a discount shop is different to that on one bought for £10,000 from Harrods.
'Guarantees' and 'warranties' are totally separate to your rights under consumer legislation. They are 'gifts' from retailers (or, more usually, from manufacturers) who can make them as generous or as limited as they like (although, of course, they can't remove your consumer rights).