This might be worth a try - it works brilliantly with wasps that have nested in the ground.
Identify the entrance, get a bottle with half a pint of petrol in it ( doesn't need to be exact amount ) in the evening when they are all in put the neck of the bottle in the entrance and cover with a polythene bag, weighted down with a couple of stones. Leave until morning. There will be a few wasps that have been out overnight but that's it. The fumes do the job, many people make the mistake of lighting the petrol !!
So for rats, the question is will the fumes kill them?