You should get a letter every year from the DWP (as you do for state pension, pension credits, etc.) with your entitlement on it. This is a good as the original award letter; if you have lost this too, a quick call to the DWP will get you what you need.
Current lease: 2025 Omoda 5 Estate 1.6 TGDI [147] Noble 5dr 7DCT
Last lease: 2022 MINI Hatch 2.0 Cooper S Classic Premium 5DR Auto
First lease: 2018 SEAT Leon Sport Tourer 1.8 TSI FR Technology DSG