.............. I have finally had it fixed !
I contacted the publisher about the "missing book cover".
They apparently fixed a problem, but Kobo still couldn't get it to show the cover so I gave them the email address of the publisher and the name of the nice woman I had dealt with and "suggested" their Tier 2 Support sort it out with her.....
I noticed yesterday that the book now has a lovely yellow book cover art, so contacted Kobo yet again
By Kobo removing the book from my Kobo account library, then me re-synching Desktop. Then Kobo adding the book again to my account library and me re-sycnching Desktop yet again....hey presto it's ok !!!
(I reckon I should make a claim for salary from Kobo considering the work I had to do to get back what I paid for )
Whilst scouring their library for more books I noticed the apparent (?) increase in these black cover books, and indeed some of the free books I have downloaded show thus not only in my library but on Kobo's website.
There are even some in their display of "recommended" books.
It's like a disease, creeping from book to book through Kobo's library.
I don't think many customers will buy a book with a plain black cover !