Class 3 Voluntary Contributions for missing years - single payment or monthly?

When making Class 3 voluntary contributions to fill missing years in your NI record, do you pay the full annual sum in one payment or do you / can you pay monthly? I was under the impression that I would have to pay monthly by direct debit, which would limit the number of years I could realistically commit to paying before the April 2025 cut-off. But if you're required to pay for each year in one lump sum then I could wait until nearer the end of the year and see how many years I can afford to pay in one go. I would appreciate clarification.

Hi Jwl,
With missing years, you would pay the year in one lump sum.
You can only pay by direct debit for the current year and going forward. 
Thank you.