LordTabby

I'd say spray the peppermint in and around the holes if you can. Same for the ammonia, or soak cotton swabs or rags in ammonia and shove them into spaces where the rats can get to

Okay I did a little digging and here are the best natural repellents I found: Essential oils (Like peppermint). Spices and herbs, such as whole cloves or cotton balls soaked in clove oil Dried bay leaves Cayenne or black pepper. Garlic or onions. Vinegar or ammonia. Marigolds, daffodils, lavender, and chrysanthemums

The peppermint spray is definitely a good idea. I thought you said you were originally dealing with mice. While I believe the same deterrents work on rats as well, it is important to know the two species apart to know what you're working with. If pest control is sent out, either tell them to back off or look for whatever lethal traps they place and clean them up. I also know that there is actually rat birth control you can get as well, it won't harm them or any predator animals that eat them, it just stops the rodents from reproducing. I'm not sure if you can get it where you're at (I'm assuming the UK because you were talking about feeding hedgehogs) I can look for a few more rat deterrents for you.

I could help you out. What's your story about exactly?

I would not recommend releasing them in the winter as they won't be able to find food or a place to escape the cold. So yes I most definitely recommended overwintering them. It would be okay to keep them until spring to ensure their survival, especially if they're showing that they don't like being in captivity, then that can most definitely ensure high survival chances come spring when it's time to release.