Well,
Car went in today, the rattle was there and the master tech came out with me... It seems jamming your fingers between the windscreen and the dash stops it , but as to what it actually is nobody knows, he hasnt come across it before. He has sprayed the area with silicone although not sure what that will do, it still seems to make noise but other than remove the dash they cant help. It seems very similar to the old 450 rattle I had in the dash and that was only cured by shoving felt near the windscreen.
Oh well, hope nobody else has this.
Other than that sailed through the MOT and a nice small bill.