We have loads of rats where I live (I think the proximity of 3 big stables and the number of people keeping chickens in their gardens may have something to do with it).
We've twice had them in the house (a semi), both times they got in where small sections of mortar had come away from the brickwork. They can get through surprisingly tiny gaps and are very good climbers, and you're could be right about them getting in via the kitchen roof.
Get some ladders and have a good look at the roof, the guttering, where the roofing felt tucks under the soffit, any points where pipes are attached etc. Do the same all around the kitchen wall. Sometimes, you can see a sort of grubby mark that builds up where they keep coming and going (they have very oily coats, apparently). If you can identify and block the access points, that should stop them getting in.
The other possibility is that they are getting into next door, and coming along the wall cavity into your property. If this is the case, you will have to get the council involved as the owner of the adjoining property will have to remedy that.
I'd also recommend that you check the pipes under your sink and to the washing machine for signs of having been gnawed. Last time we had them in the house, I woke in the night to a loud hissing sound. When I went downstairs to see wtf was going on, I found a fountain spouting away under the sink and the kitchen ankle deep in water.
The fuckers had gnawed through the plastic hose that takes water to the diswasher and it was gushing away like Old Faithful.
Afterthought: as you're in a terraced house, it might be an idea to inspect the roof space, too, in case they're coming in that way.