Thanks. I had thought about making a cutter, especially as I mainly deal in aluminium, but that means making one for each radius.
This one looks interesting as it can do concave as well as convex and bolts into the existing toolpost. I know I could use bull nosed end mills but a collection would be needed.
http://www.chronos.ltd.uk/acatalog/Lathe_Ball_Turning_Attachment.html
What do you reckon?