Experts' reports
After six months of ownership, it's up to you to prove that the problem was there when you received the product, even if it has taken until now to come to light.
So, you may need to prove that the fault was not down to ordinary wear and tear, or damage you caused, and that the product (or a component) should have lasted longer than it did.
To do this, you may need an expert's report, for example from an engineer or a mechanic.
Always try to keep the cost of any report proportionate to the value of the claim and, if you can, try to find an expert that you and the seller both agree has the necessary expertise.
https://www.which.co.uk/consumer-rights/regulation/sale-of-goods-act