Thanks for the replies Chaps, its good to see how you are getting on with your covers.
I can see the covers with the tie straps are going to hold it in place, but I don't want to grovel around underneath tying them.
I'm moving my convertible out to the front of the house and aim to MOT it, so it will be in the elements for a while, plus I'm going to be finishing it off and test drives etc so an easy to get on and off cover would be best, but I can see now there doesn't seem to be one out there.
I did buy a blue one from Argos a few years ago (they don't sell it anymore) and that was fully elasticated and a tight snug fit and never came off, but it only lasted a couple of years.
Dave