Best to do it once you move out so they cannot retaliate. When I lived in Ireland none of my many landlords was registered. Not a single one. So I reported all of them except for one (who gave me the place overnight when a homophobic landlord kicked me out, also overnight). I hope they're all registered now and strengthening the economy with the hard labour of owning necessities, refusing to release them to those who need them and scalping the prices. 💪