Its handy to get an on-line forecast but there is some way to go before it is perfect. I have 33 years of NI most of which was my time in the RAF for which I was contracted out; I am assuming my time in a supermarket, behind a bar and a season at Butlins was contracted in. Currently I am forecast to get 119.81 out of the maximum 155.65. It does state I was contracted out so that my amount is less than someone who was not contracted out but it doesn't say by how much. Neither does it say how many more years I actually have to contribute (I am currently fully retired) to get to the 155.65. I can buy year 13/14 for 704.60 and 14/15 for 722.80. So I can't do any informed maths as to whether it would be worth spending an unknown number of 700 quid lumps to gain an extra 35 quid a week when I am ancient.
It must be time for a beer......