Thanks Aaron and David.
At the moment my customer is ok with the DDR mega menu that I have created with standard vertical menu for root and first level menus and then switch to typical mega menu for second level menus in 4 column horiontal mode.
I can see it is a bit restrictive and at some point it may become too restrictive if they change their needs...but at the moment it is working for me.
It is based on the Mega2DNN Tokenised DDR Menu.
If it is useful to anyone else, here it is.
Download DDR MegaMenu
Screenshots showing it in horizontal 4 column mode for Level 2 pages and vertical column mode for Root and Level 1 Pages.