Thanks Bretticus
Scriptaculous is the one I've used. I thought xajax was good too but I wanted to keep things simple as I'm using CakePHP and xajax is a framework in itself FWICS.
Technically it's worked fine but some of my nested forms are not working for one user. No errors and repeatable. Yet I use the same browser, same OS. Go figure.
So, rather than try to fight that I thought I'd review my strategy and see if I could make some improvements that might fix it along the way.