I've just reached this stage but have hit a bit of a snag 🤪
This morning I received my response letter, with the shortfall tables, giving a breakdown of how much I need to pay in class 2 and 3 past NICs for each year, for the past 15 years. For some years, I have to pay a mixture of class 2 and 3 due to short periods of unemployment.
I've read the site you linked to, and the corresponding one for class 2 NICs, but can't fathom how I inform them of which year or type I want to pay for.