I recently paid 15 years worth , they let me go back to 2006 , it cost me around £13,000 , i estimate i will get my money back within 3 years with the increased pension so its like 33.3% on my investment. I strated to receive my pension in February thus year, i will never get the full pension as i only have 32 full years instead of 35 needed to get the maximum.
What annoyed me was i had yesrs in 1982 and 1983 and 1986 with just one or two week missing , i proved this wasn't my fault but they wouldn't let me pay back those odd few weeks.