Anyone who worked in the SERPS era was more than likely contracted out of the state scheme, which will change the number of qualifying years required. The Future Pension Centre told me that the number of years for a person to hit maximum could be anything between 35 and 47 years - in my case 38 full years would be needed.
The new state pension figures are much easier to calculate, each year buys currently £.5.29 pw (1/35 of £185.15). Pre 2016 amounts depend on how many contracted out years you had, in my case each pre 2016 year bought around £4.73 pw, requiring an extra 3 qualifying years to max things out.
Obviously it's worth calculating all the years that can be paid between 2016 and retirement age first and then making up any shortfall from the pre 2016 ones. Class 3 payments are well worth the money, and you got an absolute bargain with the class 2's.