Thanks for your comments.
I am currently using the latest firmware (1.05.11130) - and the issues also occurred with the previous firmware before I updated last week.
The ereader is not rooted (although that's debatable at the moment
As for the rotating arrows ... I don't get the status bar showing as the book (and it is not always the same one) opens to a page and then then the ereader freezes. Mind you while playing with the ereader now to check, I managed to confuse it enough that the whole screen went black and I had to soft reboot.
Assuming it is a bad epub, is the best option is to remove the most recently added books and see if it fixes the problem. I will also take a closer look at Calibre's check epub
plugin to see if I can track down the faulty book.
I will be relieved if it is a faulty epub and not a faulty ereader, as with the holidays coming up I don't want to be without it (I have some serious reading planned!). It is still under warranty for a couple of weeks, so I hope to get this sorted out asap.