They're unlikely to invalidate your policy in the event of a claim, DM. While the small print might entitle them to do that in the event of a material non disclosure the materiality of said disclosure would have to be determined in legal terms. Assuming you pay about 2 large there's no way it could be completely invalidated, it would be proportionate at worst. In practice it doesn't suit them to redirect claims to MIBI and, beklieve it or not, MIBI don't accept liability without putting up a fight. I'm aware of several cases involving non disclosure of penalty points and convictions where said non disclosures either weren't discovered through lack of interest/investigation on the part of the insurer or apparently completely ignored when claims arose. I say apparently because it's impossible to ascertain if said undisclosed non disclosures impacted on future loading, loading being that magic number they can dream up to compensate themselves for anything they think they should be compensated for including their initial decision to indemnify, taking that term in it's loosest possible sense.
However, you do have to answer all the medical questions honestly when you reactivate the policy or at least at your next renewal... that may or may not impact on your policy.