Found that the small closing panels at the top under the front wing (drivers side) has rotted through.  So I've decided on a plan!   Since it's a void and welding up their would be a pain in the   I decided to try and use expanding foam from a tin into the area pre lined with waxoyl and flatten the face with a piece of metal.  Then I'll paint it with underseal!
 
Any thoughts?

Come on Rog, look at the date of the OP - it worked  We won didn't we.......