Thanks for the replies. I've tried https://makeprintable.com/ and the site looked promising, very slick but their default settings removed all of the detail from the Object. Josh from the site has been in contact and says they'll be adding in option sliders to make the process better, I'll keep an eye on it. I've also tried meshmixer and blender but so far haven't hit on the right options to fix the shells. Lack of time to read up in details so if you have a quick "how to" available it would be greatly appreciated.
Progress happens and it's time to move on. Having taken a few weeks break I find that the Microsoft STL fixing service no longer works properly. For a start, all output now is in 3MF format, no issues there ... except that Cura 2.1.3 can't open the repaired files I download. Not the end of the world as I can load the 3MF files into Meshmixer .. where I can either resave the 3mf (and Cura will open them) or convert to STL. BUT The 'fixed' file still has 2 distinct meshes in it that end up with holes between them when I print. 2 Meshes is way better than the 25 I uploaded to microsoft but