Four observations.
1. The duty of disclosure arises at each renewal because motor insurance is an annual contract. Although your policy number stays the same and you don't fill out another proposal form it is like a new insurance each year.
2. FYI a "material fact" is one which could influence the mind of a prudent insurance underwriter in deciding whether or not to accept a risk and, if so, upon what terms and conditions.
3. The decision as to whether or not a fact is "material" should never be taken by a proposer for insurance (or an existing policyholder at renewal) as this is a judgment for the insurer to make. If you don't like their decision you can always argue it out.
4. If you are in doubt about the answer to a question relating to material facts always disclose the fact at issue. The same applies if you are aware of something else that is not specifically questioned on the proposal form but which might constitute a "material" fact. Failure to disclose leaves you open to a repudiation if something goes awry as for instance with a claim. If that happens the refusal of indemnity may itself become an additional "material fact" to be disclosed to other insurers in the future !!!