Some additional info having searched these forums a bit more, I followed the advice here as it seems to be the same issue:
https://www.mobileread.com/forums/sho...obe+drm&page=2
I can confirm that in my 'about' section it says I am
not curently authorised to view DRM content on this computer and following Sony's help page about authorising just puts me into the same loop as described above, put the password in and nothing happens, no errors, but no book appears either.
Oh, and following the 'help' in Reader, saying to connect the device, means that 'sync' has wiped my book collection from the reader too, so I've gone from not being able to put new books on to having nothing on there at all and no way of getting them on! Argh!