There are a couple of products on the market sold as duct insulation for boat heating systems. While they are at premium prices they are waterproof and reportedly designed for the marine environment although I suspect that only means they print marine on the label and treble the price.

    Eberspacher’s Maxitherm insulation

    Thinsulate’s Thermoduct insulation

    Another option as Rick has already indicated would be this from screwfix
    Screwfix insulation

    You would also need some foil tape to close the joints
    Screwfix foil tape

    Let us know which you choose and how you get on as I am sure there are a lot of other owners who would be interested in the cost and result of insulating the ducting.