I don't believe that figure for a single instant, but nonetheless, if people are genuinely being prosecuted it's because they have unwittingly let enforcement officers into their home, and either admitted to, or demonstrated that they are indeed watching live TV without a licence. No media source is going to highlight this either, because none of them would want to antagonise either the BBC or legislators for fear of becoming subject to litigation or censorship themselves.
Capita have, quite literally, no other method of enforcing this. You are not under any obligation to comply with their letters, under no obligation to entertain their visits, under no obligation to allow them access to your home, and no magistrate or sheriff is going to grant them a warrant simply because you did not invite them in when they requested.
There are no such thing as "detector" vans or equipment. Even the letters they send out if you happen to inform them by email you do not require a licence and then later sign into iplayer using that same email address are nothing but empty threats, because licences are tied to postal addresses, not email addresses, and it's utterly impossible for them to prove that it was you using that email, email addresses are recycled, its also legal for someone who owns a licence to watch at your living address provided they are not directly wired to a power socket or aerial socket, and they could well have perfectly legally logged into iplayer using your email address.
Their threats are nothing but a bullying tactic designed to frighten into compliance, and if you do not incriminate yourself, there is not a single thing they can do to prosecute.