Cheers for the replies gents.
I hadn't thought about the air coming up through the fork legs but I think it's a mayor factor.
I'm going try some spacers between the screen and the head light to see if I can tip the screen a bit and get some air running behind it. I think I will have a go at blocking off some of the air 'passages' behind the head lights and up the fork legs. I like the size and design of the screen so don't want to change it just yet. If I can pick up a cheap one I might drill a few holes in it and see how it works