Hi Scouse, are we talking about commercial premises, if so you would be better to seek the advice and a visit by a reputable Pest Control Company as there are legal issues etc, you can buy very good rat poison from most garden centres look for a product called "Sorexa" or for mice "Sorexa CD" these are professional products and the best you can buy on the open market. Traps are not really the answer as generally they will only catch the immature rats and mice leaving the adults to carry on breeding. You could also maybe try and limit the gap under the door to help to deter rodents, preferably with a metal strip.