If anyone's interested... I've found the solution!
It was right under my nose on this forum all of the time...!
To allow the completion of a book to be shared on Facebook, use the javascript from here:
https://www.mobileread.com/forums/sho....php?p=1929488
to edit the mobi file:
Record "113 ASIN" to the ASIN from the book from Amazon's kindle store &
Record "501 CDE type" to EBOK
Many thanks to gluggy, you're a star!