craigyd,
Firstly, it is a legal requirement to log your flight times. It does not however say anywhere in which format you must log them. It is only a requirement to keep a record(write them on the back of a fag packet if you wish!). I have the same old scabby FAA Jeppesen logbook which I have been using since I gained my PPL several years ago. I have since recorded all of my hour building, CPL, MEIR flights etc in that same logbook and the CAA have been perfectly happy with it. They have no reason not to be.
If you still wish to go ahead and buy a JAA compliant book then simply start recording subsequent flights in the new book. There is no requirement for you to transfer your previous flights across.