A former partner was confronted with the rat problem a few years back.Living next door to a restaurant didn't help; the rats got into our cavity wall,and we could hear them moving about in the bedroom roof,and in the kitchen roof-cavity,trying to scratch a hole through to the food cupboard !!
   She ( gun-hating Buddhist partner) summoned a Council "Rodent Control Officer" who charged a fortune for little black boxes with "Warfarin" inside.Waste of time & money.Ditto a private "Pest Elimination Consultant",again with expensive black boxes and inhumane ( Warfarin causes rats to slowly bleed to death internally--Yucch !) poison remedies.
Again,zero results,and waste of money.Finally,she allowed me to buy some rat-traps ( just like mouse-traps,only much bigger,so DON'T let your fingers get caught--it HURTS !).I baited these with Edam cheese,laid them along the edge of the patio/garden wall,and killed 2 or 3 rats every night for a week or more . Problem solved !!
            I am also happy to shoot them ( and rabbits,pigeons,or grey squirrels ) if you can't face it yourself---I live in Oxfordshire,and could do with a little "retirement" job !!! OK ???
    I will NOT,under any circumstances,use,or advocate the use of,any kind of poison or other chemical method of pest control.Armed Forces firearms-trained on .303 Bren Gun,.303 Lee Enfield rifle,.38 Webley revolver, 9mm Lanchester sub-machine gun: Showing my age,I think !!