The "no licence required" thing is just yet another sham, and nothing more than a tool they use to ascertain who, precisely, is living at an unlicenced property.
They send all mail to "the legal occupier" until such time someone responds and confirms the details of who, precisely, "the legal occupier" actually is. From then on they will harass the individual in no different a way to they harass all unlicensed addresses, but with the difference that now they have a specific, named individual, they will likely appear on the doorstep asking for that person by name and subjecting them to the "can I just come in to check..." crap.
Some people are lucky and the declaration seems to prevent the letters for a year or two, but far more are reporting that they never once saw hide nor hair of a Capita employee until shortly after they filled out the declaration.
As always, the best approach to communications from Capita, whether it's in the form of a letter or an appearance in person, is to completely ignore and rebuff any attempt at communication and correspondence from them whatsoever.
Do not acknowledge or respond to their mail fishing attempts, do not converse with them on the doorstep, do not identify yourself, and for god's sake don't entertain any request to enter your property unless they are presenting a warrant, which is vanishingly unlikely to happen if it's their first in-person attempt to gain access, because no Magistrate or Sheriff in the UK is going to issue a warrant simply because Capita suspect you might be watching TV.