Thanks for the feedback!
OK, I'll give you some more details..
I got the idea from here
http://www.blackanddecker.com/projectcenter/DocumentView.aspx?DOC_ID=p_2_102_23915_23954_24035.html
except that the curved surface I am constructing is concave rather than convex as show here. Its for the edge of some decking which is built on 2"x6" joists which rest on a layer of gravel.
So I was going to use 1/4" ply, probably 4 layers, then finish with a layer of 3/8"ply, if necessary (not sure why the last one needs to be different, but that's what it suggests at the above website).
Was thinking of using Evostik Weather Proof Wood Adhesive with screws - is this ok, or can you recommend an alternative?
The minimum curve radius is just over a 1m, but the curve traces out an ellipse. Total length is under 7m, with a height (with the ply resting edge to the ground) of 150mm. I was hoping to get it all from one sheet.
PowerTool, what is your suggestion re. flexible MDF? Is this better than plywood in damp conditions?
sidecar_jon, thanks for the boat building tip. How would you do this for a concave curve?
Thanks again for your help and advice!