Thanks for that link.
Yes I am "middle aged" as my 91 year old father calls it, later this month. I saw you could make extra contributions but thought I was getting the maximum. I think I had 33 years when I calculated it. Somewhat surprisingly, I didn't receive the pack you are supposed to get a few months before so I think I had better have a phone call to pensions . It might be because I was contracted out while I worked for a Blue Chip company. I asked for my S1 at the same time and that hasn't arrived yet( A DVD bought on UK ebay on 11th Jan turned up today........)
If you have 30 or more pre 2016 NI years then there is absolutely no point in trying to buy more pre 2016 years because you are already at the maximum of 30 years. DWP will happily take the money if you attempt to pay, but it won't increase your entitlement, and they won't refund it when you realise your mistake.
Buying post 2016 years is generally worth it though. They add 1/35th of the full amount per year purchased, so £4.56 per week (£237 p/a). You should be able to buy 2016-17. You cannot buy the year you actually reach SPA though, so if you reach SPA before 6/4/2018 you cannot buy 2017-18. If your birthday is on/after 6/4/2018 you can buy 2017-18 as well, which will add another £4.56 per week.
If you live to the same age as your father you'll get and extra 1352 payments ((91-65)*52) of £4.56 in that period, which is over £6K, for a cost of £735 ish.