It was the same with a lot of big firms, at our retirement seminars they said we needed 35 years of NI they did not mention that some of our years were contracted out.
For example I did 40 years with the same firm but still needed another 6 extra years.
I was told I could not buy any years because I had worked throughout my adult life. I worked and paid NHI for 50 years yet received a reduced pension. But I am not complaining, I have my fire service pension and my OAP. I reckon I have been lucky, far luckier than others, who have worked hard all of their days and have much less to show for it.