Sorry guys, didnt actually know I had replies to this!
Thanks to all for the speedy info.
@gainsy - I have no idea mate, but for me, I simply like them because they are only £1 extra and they require no cutting whatsoever, so they are a perfect fit. I always end up hacking my foams either too much or not enough.
Maybe someone who knows about such things as racing could shed some light on if these are actually "better" in race terms than standard foam?