The inflation measure used is RPI which is more difficult to find as it's not so commonly used any more. If you go to the ONS website you can see the latest value for RPI for December which is 2.7%
Using the existing FIT rate of 52.75p and applying the RPI of 2.7% gives a new rate of 54.17p
for the Feed in Tariff rate as of April 2019-2020. The level for export will increase to 3.82p
There are various other FIT rates that apply depending on the date that your solar panels were commissioned. They will also increase by 2.7% as the RPI rate for December 2018.
|FIT Rates April 2019 Onwards|