Technically yes, you have a duty to notifiy the insurance company of any 'material changes' to the policy. I ran into this a few years ago, but I was only 10 days away from renewal and I wasn't going to go with the company I was currently with. I rang to see if there were any charges and they wanted £35 to take a bike off. I said but it's only 10 day until the end of the policy, can I not let it just run out ? They said no it has to be taken off the policy. Fair enough I said, I'll do it by post. Needless to say the 10 days passed and I heard no more from them...