Since they are not the government, they cannot legally gain entrance to your property (actually, neither can the government without a warrant, but they cannot g

Since they are not the government, they cannot legally gain entrance to your property (actually, neither can the government without a warrant, but they cannot get a warrant either). They send you a letter, asking you to sign up to pay the media licence or sign a declaration that you own no such devices. The declaration - however - is legally binding and there are huge fines for breaking such a declaration. However, they must be able to prove that you do actually have such devices, by entering your property. In most cases, they don't really bother, though.