Try and find if you have a proofing issue external areas,holes, trees /ivy up against the property, look for a access point , if you have adjoining property/ house ask to have a look there as well, if it looks as though the loft is the base for the activity ie nests are in situ there is probably a external food source as it unlikely one house would supply enough food for a full blown colony. There has to be a common link for the activity site and food and water supply ie bird table, water and an access point(s)
Initially install fenn traps using a couple of different baits, if you find they are frequenting a bird table, use bird food in a paste on the plate of the trap you must cover the traps with mesh covers on the run lines through the loft and place sand or flour to find most used runs, importantly check every day, trapping will take longer but you see the results, when you have removed rats - check this using the sand method, close up any proofing points, I wouldn't advise proofing immediately as you could force them into the kitchen for food and drink.
Good luck
MD,