Thanks all for your helpful comments, glad I asked.
A small square of the ply laminate about a foot square has come loose, so I'll just pva glue it back in position, I won't under seal it or any of the floor then.
Cheers folks,
Regards Brad
Hi , I have an Elddis avante 524 2009 , I am proposing to under seal the outside face of the floor underneath the van, does anyone think this not advisable for any reason before I do it?