One thing to note is that Nominet sent an email the other day advising that the Nominet 2.0, 2.1 & 2.2 schemas are being withdrawn in June 2013, so I'd not code anything new against these.
Also, Nominet seem to require the XML document to be in the exact order the documentation is in, this caught me out for a while last time I made a change.
Additional thought: Send an email to Nominet support, they do have access to some people who do understand the inner workings and are really helpful (well they were last year when I moved away from the Nominet 1.x schemas)