Monitor your inverter/s online. This is the simplest way to keep an eye on the health of your solar PV. Inverters create error reports for any issues detected which is invaluable information when reporting a fault to your installer.
Worth bearing in mind is climatic conditions which vary by location, throughout the year and year to year. 2020 saw the UK enjoy 1495 hours of sunshine which is 109% of the average whereas 2021 was 1391 hours and 99% of the average.