All you have to do is make sure nothing of his is registered to your address.
Wherever he actually does register them is not your problem.
As another poster said, every time you get a piece of mail for him just put a line through the address window, write "not at this address" on the envelope and put it back in the post.
When you do speak to them just explain that you don't know what his official address is. Just be honest, it's extremely difficult to prove someone is lying if they aren't!
You have all the proof you need that you are living alone which is bank statements, bills etc all in your name only.
I got straight through on the 03000 number at lunch time on Monday. I know it's not much consolation but the lady I spoke to was lovely.
And don't worry about the time limit. You only have to speak to them by that date, if they need you to send anything in they'll give you another date to send it in by.
I know it's scary, I spent the weekend in a right old state but honestly, you'll be fine.