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Fenn mk4 traps, they won`t work for about a week but after that they work 24-7-365.The reason they won`t work to start with is because they hate new items in their area ie the trapping tunnel etc.As for the rats in the roof !!!!!! get them out as they are a major fire and flood hazard :o :o :o
I don't think much of the Fenn Mk4 traps.See: http://www.antisnaring.org.uk/news_wire/angmering_park_fenn_trap.html
If you want to poison them, I'd suggest lots of avocado skins. Cheaper than poisons, leave the skins where the rats can get at them with a thin layer of the avocado fruit still in them, and when the pests eat them, they will die in short order. Just make sure the dog or cat doesn't eat them if they are about because Avocado skin is toxic to just about everything that eats it.http://en.wikipedia.org/wiki/Avocado#Toxicity_to_animals