Shooting them in the loft is really difficult. How much time are you prepared to put in waiting for him to turn up? - and can you get a clear shot anyway?
Rat poisons cause desiccation of the corpse so there is no smell, but I don't think you can get the good stuff unless you are licenced vermin controller.
Trap is by far the bet option. Killing traps are cheap - live traps are slightly dearer - take it outside and dispatch at your leisure.
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TANSTAAFL