It was certainly true in my case, but I think it helped that I chatted it through with my desk officer and had my boss onside as well.
The NI gotcha is quite easy to overcome with a letter to the NI contributions office - P2 should be able to furnish the address. As I recall, you nominate the income you want NI stopped from, on a government form of course, and hey presto, it all happens. They follow up every few months to see that the situation remains as you described it. Tax, however, is another problem. Write to your tax man and let him know the situation.