At first we thought it could be mould but we don't think it's this.
I've included some pics for you to take a look.
http://i683.photobucket.com/albums/vv19 ... ci/126.jpg
http://i683.photobucket.com/albums/vv19 ... ci/124.jpg
What could it be because we are stumped - it looks so awful on our nice new deck and we are hoping it's not permanent!!

I've also posted this in the General Discussions area for more traffic as we need to resolve this problem ASAP.