Ok so the solution ended up being use Akeytsu to delete the UE4_Skeleton Node.
Then I could use the Skeleton editing tools to disconnect the Pelvis, then set the Root Rotation to 0,0,0 then I hit Accept and that seemed to do it.
(I did not seem to need to reparent the Pelvis to the Root, it happened when I hit Accept. (I would have assumed I would have to do reparent it))
Other things I tried were rotating the Skel mesh in Blender- that brought in Skeleton with huge massive bones, so to fix that, then I tried using the Unreal Blender Add on - Send to Unreal. You get it from Github and watch the video here, carefully to set it up. Various Unreal plugins need applying.
In the end I needed to change the default Push Settings from Blender to Unreal like so - otherwise my skelmesh came in upside down with an extra bone added.
So another whole day of trying hundreds of things out, because like always with mocap if something, can be rotated 90 degrees on X, Y or Z then it will be with 80 combinations that are just wrong, an no obvious way to fix
