Thanks for all your advice. I managed to get my H20 connected to calibre and transferred 599 books (according to Calibre) after removing the SD card. Unfortunately the front screen says I have 481 books (but I think that includes the non downloaded Kobo purchases) but the settings say I only have imported 158 books.
So as you can see I am totally fed up and confused.
I'm going to park the kobo in a drawer and forget about it, it's far too much trouble, I'll go back to reading on my Kindles. I won't ever by a Kobo ereader again.
And to add to the mess I have two kobo books in my library on my device with no download button. According to my kobo website library one is listed, but I need to rebuy it, and the other isn't in my kobo website library at all. I know I bought both books from kobo as I have the downloaded ePub safely stored but I can't prove it as my purchase history will not display, I just get an error message.