Ok..the Key was your Quadrige decimation only in Meshlab then Instant Meshes! :)

Barry - how are you cutting off those pieces from the surface? Thanks, Andy

Hi Andy,
I offset the surface then trim inner and outer surfaces.
Then selected naked edges of the trimmed surfaces.
Apply join command this creates 2 curves I then rebuild them.
Loft these curves and join to trimmed surfaces thus creating a solid.
If you need more detail let me know.
Cheers
Barry