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.