If you're not allergic to wool, consider scavenging at a thrift store. There are usually sweaters or suits or something that can be reworked.

Oh, good thought! If I could find men's trousers that were big enough around the waist to tuck a big jumper into them, I wouldn't need to convert them, I could just wear them with suspenders. That would be a lot less work! But I'm getting the flannel anyway because it's only £6 a metre, and I will definitely use it.