Thanks for the reply

I've actually managed to put in the line I wanted down the average
value by reseting the y-axis to cross the x-axis at the average value
rather than the 0 default - this has worked well

So basically I've now got an in-sheet method based on 4 cell based
formula per sensitivity parameter which I then feed into a stacked bar
chart - and then I reset the y-axis crossing

This produces a nice looking Tornado plot without the need for an Excel
add-in

I'll post this as an example when I get time to tidy it up a little

Steve