you can blow £70 on a B+W 110 10 stop screw in filter or you can get a hitech 3 (10 stop) ND 85mm square filter that fits into a cokin P or kood P filter system.
http://www.formatt.co.uk/stills-fil...-d/high-density-neutral-standard-filters.aspx
http://www.formatt.co.uk/stills-fil...-d/high-density-neutral-standard-filters.aspx