You would need to contact DWP Pension Service to confirm if paying additional years would increase your pension entitlement, if so they would also confirm what years you would be able to pay. You can find contact information here:
Contact the Future Pension Centre
If DWP do confirm you can pay additional years you can then contact the National Insurance helpline make payment.
National Insurance: general enquiries
You can find more information on Voluntary National Insurance here:
Voluntary National Insurance
If you decide to make additional payments your pension would only increase from the date you made the additional payment. This would not be backdated to when you initially began claiming your state pension.