This is an inflation index published by the Office of National Statistics
Increases to pension in payment
Once your pension commences (from your normal, early or late retirement date), each 1 April, it will be increased annually in line with the annual increase in the Retail Prices Index (measured in the previous February) capped at 2.5%.
As an illustration, for existing pensioner members (for benefits earned post 2005), the pension increase awarded on 1 April 2020 was 1.05%.