Thanks for the tips so far, I will try it later. The weird thing I've just noticed is that the PHP runs fine and responds with the correct messages (XML - Added ok - etc etc) but the ajax error function fires, hence why I want to see the exact message.
I will try try alerting xhr.responseText and see what I get. And I've read this: "from the source: (Function) error - A function to be called if the request fails. The * function gets passed tree arguments: The XMLHttpRequest object, a * string describing the type of error that occurred and an optional * exception object, if one occured. " But this is what prompted the question, I don't find some of the docs particularily helpful to newbies, unless you understand the JS behind it in the first place which I am getting there with! Thanks Simon