Originally Posted by davidfor
<snip> you need to do an epup->epub conversion to add it to the book. </snip>
Yep, thanks David that did the trick - I used Calibre to convert the original epub with added cover page to an epub. Then after downloading to my Arc, the covers were visible. The only trouble is that the Kobo website has now become convinced that I no longer have - and never had - these books; it has removed them from my library, and even removed them from the "deleted" list.
And the "synchronise" function on the Kobo App seems to only work one way - it certainly hasn't worked to inform the website about the books that I have on the Arc.