Thanks for the info John, but I've now bought a cover from Airflow UK of Oxen. It's two layer, breathable outer and, I assume a breathable, fleece inner. Cost £88 posted to my door.
Just fitted it today. Elasticated both ends and two straps which go behind the front wheels and infront of the backs. The fits not great, but this does leave room for mirrors and air space as well. I'm not convinced yet as I think it's gonna be loose in stormy weather, but time will tell :?