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I am UK resident. I have not paid any NI for 2023/24. I want to make a voluntary Class 2 NI contribution for that year of £175.40 (ie, £3.45/week). But I am not being offered the option to pay it online. My NI record for that year says "Year is not full" and "We are checking this year to see if it counts towards your pension. We’ll update your record when this is finished, you do not need to do anything." As I say, I know that I have not made any NI payment for 2023/24. How can I pay £175.40 for 2023/24 (bank account details etc) and what 18-digit reference should I use?