How to pay Class 3 National Insurance for specific years only?

[whereisthejasmine]

I have some years where I do not have a full year of NI paid. On clicking the details it tells me how much needs to be paid to make each year a full year. some years it is less than £100, some are £800. I would like to pay up for the years that are cheap. I can see you pay by bank transfer and use your UTR as the reference. But I cannot work out how to pay for particular years and not others - can anyone tell me how this is done?
I have had part time jobs and have been self employed for all of these years.
I would like to pay the class 3 NI for the years almost covered by my employments and then in additon pay class 2 NI for the other years. Again I cant work out how to do this. All of the years are within the 6 year deadline.
Very grateful for any help.

You phone HMRC and they give you a specific unique reference for each year. It’s possible to work our what it would be - include your NI number jumbled up and the amount - but I feel safer asking them.

It can take several weeks for the money to be shown online, so they tell me. I’ve been paying added years as they pass.