The only hard part was getting ahold of the NI registration services in the first place, and that complication was caused by a fit of bad timing on my part crossed with a change of systems on theirs. After that, it was all very straightforward.
The last part of the letter with my number in it irked me on pricipal however. After giving the boilerplate "if you change your name or address let us know" information, it goes on to say, "If you are a woman, please also contact your local Social Security Office if you get married, get divorced, are widowed, or have your marriage annulled." Not if you're anyone at all. Only if you're female. The system isn't quite where it could be yet.