Melchy,
The obvious way to do it would seem to be to go back and re-build your pension year-by-year from 2015 to 2022, taking into account any promotions as they happened, and work out the tax due at the end of each year through that period. Add up the tax due over the 7 years, compare it to what you actually paid, and bill you for the difference.
I can’t see them making a huge one-off adjustment to your pension value in 2022 because that would certainly cost more in tax than a series of smaller back-dated adjustments. And that would penalise individuals for receiving in a lump what would have been given to them incrementally had the Government not acted illegally.