Thanks to all for the info, have wired each side of the screen to a separate relay, with no timer. Will just have to remember to switch off after use!! Also found you might need to check the location of wires coming from the screen, looks as though they can get trapped by the bonet hinge - used a dab of silicone to keep out of the way.

For relays used a 40A one from the banks of those at bottom LHS footwell area, behind glove box.

When it came down to it, didn't need to use the damn thing, sunny all day, which was good for the marshals.

Roy.