Ignore the above advice. I have had this problem before and needed to prove that I had more than 9 years or face a hefty increase in price. My problem was because of a insurer only stated I had 9 years instead of 14 years and my new insurer, Aviva if I remember right, wanted proof or they would increase the premium quite considerably. It was a nightmare to sort out as I had to go back several years to previous insurers some of which at first weren't keen to provide the letter I needed but prsistaance paid off in the end. I had to go back six or seven years to get a NCB confirmation letter then pass it on to the next insurer who would then give me one from when I left them and pass that on to the next and so on. It took me several days and a load of phone calls and emails to get my no claims back to where it should be and satisfy Aviva and it was a lot of hassle. I am now sitting on 20 years NCB and almost had to do it all again as the insurer I left last year, Agea, only gave me 9 years and tried sticking with "it's our policy to only recognise 9 years" until I finally got hold of someone with some common sense who sent me a letter confirming what I actually had. So get onto that insurer and press for what he is owed. If they try to fob you off keep on at them.
Liverpool Victoria done this to.me,
This years renewal I switched companies but everyone I spoke to said they only recognise 9 years, despite me having proof from previous insurance of 13 years NCB.
I fell out with LV because when I joined them I had an at fault claim against me, half way through the year I had it confirmed that actually the claim had been deemed not my fault and as I never claimed for my car the insurance company took the view that no claim was ever made. I phoned LV and I got the most useless bunch of people I've ever spoke to and essentially refused to acknowledge this despite me having a letter from the previous insurance company and would not reduce the premium.
This year I switched and saved £400 I'll never use lv again