Let them know as soon as possible.
7 years ago I was using one of the "compare the Meerkat" type sites and answered an ambiguous question wrong, I had had a claim in a car which I didn't declare thinking the question was about bike claims, a couple of weeks after the bike policy started I got a letter saying I hadn't declared the car claim so they were cancelling my insurance. This has affected ALL my insurances since then (car, bike, house etc.) as one question that is always asked is "have you EVER had insurance refused or cancelled" which when I answer yes most places won't insure me. It took me a few days phoning to get insurance for my bike and ended up paying about 4 times the original premium when I eventually found someone who would insure me.
Since then I have to find a company that has a human that will speak to me when I want insurance for anything as with any online companies "the computer says no" when I tick the "cancelled" box, unlike the "any accidents/convictions in the last 5 years" question there seems to be no time limit on the "cancelled" question.