My 8kWh battery was only giving 6kWh after 3 years. I contacted the company and made a warranty claim, which they ignored. I downloaded a complaint letter from Martin Lewis's website and cited the Consumer Rights Act 2015 and got a phone call from the CEO of the company.
After 3 years, the battery had become obsolete (which was probably why they were trying to ignore me and hoping I'd go away). The upshot is, they offered me compensation for the missing capacity I'd had to pay for on the electricity bill, or a discounted upgrade to a new 12kWh battery with new 10 year warranty. (Guess which one I went for)