I would have thought that for a open claim the Insurers would have deducted the 2 years NCD relevant to a fault claim and the adjusted NCD would be shown on the renewal notice. If they have not done this, then that is not your problem.
I would personally suggest that you should phone companies to confirm the claims details and to provide the NCD years shown on the renewal notice. The Insurance staff you speak to should work out that the NCD is wrong.
Your only problem is that the new Insurers are connected to the previous company and they look up your previous Insurance details. If they see that the NCD is wrong, they may come back asking for the extra premium. Once you are locked into a contract, you might find it difficult getting out of it, If the premium is unafffordable. They will require a cancellation fee of up to £50 plus the time on risk. It is possible that if you are caught out using an incorrect NCD, that you would miss out on shopping around for the best value Insurance at the correct NCD level.
It is up to you how you proceed in regard to getting the renewal notice corrected and therefore advising new Insurers of the correct number of years. BUT you should make sure any new company is aware of the open claim.