I don't know how or if Rentokil charge for their services, but they will come and put down traps/poison and see where they are getting in. Block up any holes in the skirting boards or where pipes run up into the property, Aluminium foil or even used brillo pads/steel wool is a deterrent.
some Neosorexa from your nearest farmers store would work wonders.....they have a lesser one for mice that is sitting up in my attic but I am not going up there now. Just throw out a few punctured packs and they nibble it and head outside as it targets their kidneys....
Thanks for the replies, Council charge if they are not in the main house, have got a local pest control firm in, same rates but I'd rather give them the money !