Hello Joanne,
You are clearly having some problem with the cushions. I do wonder why you say "althought you only get 12 months" That might be true of the manufacturers guarantee, as they can specify and limit what they like in them, but you are covered by the Sale of Goods Act, which does not limit you to 12months but to what should be a reasonable life expectancy for the product.
I feel I must add an observation:-
The fact that the upholstery has failed on several occasions, may indicate that the upholstery is not to blame. It may just be symptom of another underlying cause. Perhaps it's a faulty bed box, or may be you are not using it in accordance with the manufactures instructions.
With the above in mind lets consider it situation if the upholstery is the only thing that is involved.
In your case, the product has already demonstrated on a number of occasions it is not fit for the purpose that you are putting it to.
SoGA kicks in automatically when ever you purchase a new product from a retailer or trader. It requires the seller to supply goods that are fit for purpose. If the seller is aware of any reason that the product may not be of adequate strength or durability, they must advise the purchaser before the sale is completed, so the buyer cane either waive their rights against the product, or to seek alternative product that will be strong enough etc.
So for example, if the prospective purchasers were larger/heavier than the seating product is designed for, the dealer should make the purchaser aware of the durability issues that might ensue. They should either seek to get a written waiver for the warranty or they should seek to supply goods of an up rated nature.
Please let me assure you I am not claiming you have a weight problem, But if the dealer did not make any representations to you on this point means they must believe the product will be sufficiently durable. Your posting suggests the product is not sufficiently durable, thus the seller has either sold faulty goods or failed in their duty to supply goods of merchantable quality.
In either case, the dealer is responsible in law under the SoGA. Do seek professional advice before pursuing a case like this, because, If the fault occurred within six months from the date of purchase, the seller is obliged to prove the goods are not faulty. After six months, it is up to the purchaser to prove the goods are faulty. In your case I do not know when the problem first arose but if it was within 6 months I think you have a strong case. Based upon your reports I believe you would still have a strong case after six months, because of the repeated nature of the occurrence.