If you are using SSR numbers there is no need to register again. I confirmed this with the lake wardens this Easter, here is part of their email reply:
"SSR numbers should be displayed in the same way as LDNPA registration numbers. Only the Numeric aspect needs to be displayed the letters do not. The number should be white on a black background font is Helvetica Medium 1 inch stroke and 6 inches high. Numbers are available here at Ferry Nab at a cost of 50p per number."
As you can see they sell numbers stickers but as I wasn't launching at Ferry Nab it was more convenient and saved me time to make some up before on ply boards.
If you do want to check anything they can be contacted on firstname.lastname@example.org