When I contacted the DWP about my AVCs they sent me an email showing which years I should pay - here is an excerpt - note the warning about pre 2016 payments.
About paying more years of voluntary contributions
Paying more years of voluntary contributions will increase what you will receive for each year paid. For pension purposes it is only beneficial to pay any of the post April 2016 tax years.
The table below shows what you can pay.
If you do not pay by the first date shown, the amount may be more when you do come to pay. We cannot accept payment if we get it after the final time limit shown.
Tax Year
Number and rate of contributions
Amount
Payment date without penalty
Final time Limit
2016-2017
52 @ £15.40
£800.80
Within 8 weeks of the date of this letter
5 April 2023
2017-2018
52 @ £15.40
£800.80
Within 8 weeks of the date of this letter
5 April 2024
2018-2019
52 @ £15.40
£800.80
Within 8 weeks of the date of this letter
5 April 2025
2019-2020
52 @ £15.00
£780.00
Within 8 weeks of the date of this letter
5 April 2026
2020-2021
52 @ £15.30
£795.60
5 April 2023
5 April 2027
2021-2022
52 @ £15.40
£800.80
5 April 2024
5 April 2028
Payment date without penalty
If you want to pay your contributions, at the amount shown, you must send your payment so we get it by the ‘pay without penalty’ date. If we get your payment after this date, the amount will increase.