@Prawns, that's a very good point.
I have no idea as to how a chicken would go if eating a poisoned mouse.
I do have a bait station in a small cage now, so the chooks can't get to it.
I think I'll put a couple of traps in there with peanut butter on them instead and see how that goes.
The Yates website has this humane solution:
The Ratsak Electronic Mouse Trap is a highly effective way to control mice, using advanced smart circuit technology that senses when a mouse has entered the trap, while a uniquely designed tunnel prevents escape. Once inside, the circuit triggers a high-voltage shock, killing the mouse quickly and effectively. The electronic trap kills up to 100 mice (per set of AA batteries) and can be baited with peanut butter or Ratsak Attractant Paste.
Morning @MonkeyLearn I know I am late to this discussion but I would like to reinforce what @Prawns says. Baits are great but if you do have any animals that are likely to eat the dead mice then this is not desirable. One or two won't hurt them but if there is large numbers regularly - then I think a trap is better. YOu can get humane ones now. You should also seal the gaps in the shed where you can but you will struggle to seal them all and they will only go elsewhere looking for food.