Zakat calculation on dormant pension

Asalaamu alaykum

I recently became aware that zakat is due on pensions and am keen to understand how to calculate my outstanding to be paid zakat in light of my personal situation.

I had a private pension that I paid into for a few years from around 1999 onwards after which I stopped. I don’t have any historical statements except a recent statement showing the current value of the fund from the company that now handles my pension after the original company was bought out.

How do I go about calculating (or estimating) the zakat I should have been paying?

Would really appreciate any help or advice.