I'm so sorry. I am constantly on guard for predators, and even after I increase defenses, it seems eventually a predator gets through again. I mostly deal with raccoons and weasels here. I certainly know how you feel. Here in the USA we have what is called hardware cloth, and it is recommended a lot on BYC forums. It is just quarter inch metal mesh that we use staple guns to put in place. And that is what I try to put over the smallest openings. But I imagine it is difficult to secure anything completely against something as small and wily as a rat.