Hi Devin, Hi Bob,
I tried everything now. I created a Boot Camp partition, installed XP, brought it up to date, installed the Sony software and ADE. Plugged in the Reader, registered it with the system. Bought another book - crash after crash after crash.
I then tried a forcible reset, re-registered it, downloaded another book into Mac ADE and then tried copying the ePub file via Finder as suggested in Bob's link, but with the same result.
I'm suspecting I have a faulty Reader, so I've emailed Waterstones' never-knowingly-helpful customer services to see if I can take it in to a store and get a replacement.