For all the reponses...
I do now have 6 devices on my account, I need to contact sony support to get them to remove my mums old reader (seems odd that I can't do it myself).
Anyway, I managed to talk her through the authorisation process on the phone in fairly short order (not helped of course by the fact that she insists on clicking everything in sight half a dozen times "to be on the safe side")
Once she had authorized the new machine I had her stick in the memory card I had sent her and after the usual 2.5 hour wait for the reader to do whatever it does all the books became available for her to read.
I'll be taking a look at some of that remote software particularly now the reader has become available in the UK. Digital Editions looks much easier to use for the novice than the sony library software and gives you a prompt if you try to copy the same book onto the device more than once. So I'll probably install that remotely as well as the latest hotfix for the machine.
So
again...and have a nice day y'all