A warm draft escaping the cottage is a reverse Pied Piper, luring heat-seeking mice inside. Block entry routes with something rodents won’t chew, plus caulk if the gaps could allow water in. Xcluder is a mat of polyester and stainless steel fibres. It won’t rust and, once poked in place, expands slightly for a tight fit. If you have one or two gaps to block, you can unroll a knitted copper pot scrubber. It’s a little stiffer than Xcluder, but will do the trick.