As you can't just pop down to the Post Office anymore, and the phone line wasn't then installed, I bought it online (whilst at work). You can opt (apparently, it's recommended) to have the licence emailed to you, so I did. However, when I came to enter my Credit Card details the registered address is still at my old abode (which I am still using as an accommodation address) and I got the message "as you are not buying a licence for the address at which the card is registered you must have a licence posted to you".
I assumed that this meant it would be mailed to the registered address in order to protect against someone buying a licence with a stolen card, but no, after 10 days it plopped through the letter box at the licensed address.
Scratches head in wonder. I still can't work out what this is supposed to achieve!