Have you asked your current insurer what their letter of NCB Proof says, exactly? You might find that it doesn't mention the 10 mth thing, in which case it may not matter.
For example, I had an at-fault accident in 2013 but I've still got 10+ years of NCB, as stated on my letter.
I don't know of any insurer that asks whether the NCB is 10mth or 12mth. If they don't ask, and it doesn't state on the letter, I don't think you'll have a problem.