Thats great Roger, and very informative!
It looks like I can get away with using the 460 then, though not ideal. Buying more than one of the larger masts looks like it could have become expensive as there would be more relevance for the higher carbon content and thus £££.
I am surprised but glad I can use so much extension on the 460...I didnt want to wreck a new mast by experimenting without advice! I can see from your answer that the 8.0m sail will not function as well as it should without the 490, but I may have to put up with a compromise at the moment and I'm only 70kgs so may be ok.