galadhir

I second the recommendation of flannel, and layering. If you are sewing a dungaree style (what we in the States call "bib overalls" or just "overalls"), you could do it in whatever fabric makes you happy, though, and it should still serve your purpose. I'm imagining flannel ones, and ones sewn out of that double-layered puffy stuff, and ones sewn from repurposed dress pants..