Woozer’s advice is best (contact your insurance company and advise them of the situation).
Rip off insurance companies will want you to insure the second car, giving you no no-claims discount – claiming that you have not earned the discount for a second car. If they come this old ruse – contact Direct-Line who will give you whatever no claims discount you have earned on your first car, on both cars. The down side is that once you sell the car (and no longer require the insurance) they will refund the remaining insurance premium, less an administrative fee (£25, I think).