Hi Ianpiper
It depends how big the tunnel is. Personally I wold avoid using tunnels because many people use them as entrances to hog boxes to deter cats. If you are going to use poison, winter is probably the least bad time, because there are probably very few hoglets left around. Had you thought of a bait box, which have very small entrances. A hoglet would probably get in, but possibly not an adult. Personally I would try to go down the deterrant route first. For instance do you feed the birds, because I find it is often dropped bird food which initially attracts rats. The following are various links from previous posts on the Forum about rats:
https://www.hedgehogstreet.org/forums/topic/hedgehogs-and-rats/
https://www.hedgehogstreet.org/forums/topic/rats-2/
https://www.hedgehogstreet.org/forums/topic/rats-3/
https://www.hedgehogstreet.org/forums/topic/rats-4/
https://www.hedgehogstreet.org/forums/topic/rat-poison/
