I went on to the Government Gateway site a couple of years ago and noticed that in one particular year I had 51 weeks of contributions and the year didn't count. I paid for the extra week (can't remember how much, but it wasn't much) and as soon as it was processed, my projected pension went up by just over £4 a week and my record for that year showed as complete.
As I explained above, I can't complete my final year before my 66th birthday because the rules don't allow for it, as my birthday is before the end of the tax year. I could pay for the months before my birthday, but they wouldn't count because I still wouldn't have a complete year. If I were employed, I'd have to pay for them anyway, but they still wouldn't count.
MOnica, it's possible that the rules have changed with the new system. I honestly don't know, but that's been my experience and it's been confirmed by telephone conversations.
I have 47 years of contributions. Some of those years were opted out and some weren't. The opted out years can't count towards a full state pension (only the reduced rate), but the years when I wasn't opted out do count, including the last six years when I've been self-employed and have paid voluntary contributions. Each year I've paid extra has counted towards increasing my state pension by over £4 a week.
For people who haven't reached state pension age yet, it's well worth going on to the Government Gateway site with your NI number and getting a full breakdown of contributions. Any gaps can be made up and it's not difficult to work out whether it's worth it.
Another point worth remembering is that people who received Child Benefit after a certain date will have their qualifying years reduced.