OK. Things are looking up
Just managed to buy a book
My Kobo is the Kobo Wireless eReader version 1.9.10 and usually I don't have any problems with it. I bought it in November 2011 - was being sold for £49.99 at W H Smiths at the time and since I'd never had an eReader before I thought I'd give it a try.
What I did was uninstall the Kobo desktop, reinstall it then downloaded the Kobo App (downloading the Kobo App which seems to be the latest thing was my son's suggestion - he's a bit of a computer geek - keeps my temperamental laptop working).
Seems to have done the trick. Decided to try and buy a book and it accepted my card no problem this time.
Maybe their site was having problems last night (was about 2am UK time when I tried to buy the book). Maybe that is not the best time to buy.
Anyway, happy bunny now - with a book to curl up with later tonight