Why would paying monthly help?
If you are setting up a policy for a year, paying for it monthly is just a way to spread the payments. The insurance company still expect the full amount to be paid overall for the year and then refund the amount left, minus the cancellation fee, admin fee and whatever other fee they can come up with.
I would try your current insurer as KykeR suggeats as they can sometimes provide temporary cover like this.
Because he said he doesn't want to fork out £600 only to get half of it back. If you have paid it all up front, you have little control over how much you get back, but if paying monthly (and your insurer doesn't require a bumper deposit) the worst they can do is not refund a couple of week insurance you've already paid for that month
You don't really have any control over the amount you get back from a cancellation of a yearly policy, it's set out in the terms and conditions for that policy. If you cancel your monthly payments and refuse to pay the balance they will most likely just pass on your details to a debt collection agency to retrieve the remaining amount (that usually adds a fee). Not really worth the hassle is it?