Pour about ¼ of the hot milk into the creamed eggs and flour and mix in, then whisking continually, slowly add the remaining milk. Pour into a clean saucepan and bring to a simmer while whisking, and cook for a few minutes until thickened and custard-like. Pour the mixture through a fine sieve into a clean bowl. Cover with clingfilm, letting the clingfilm touch the surface of the crème pâtissière (this will stop a skin forming). Cool, then chill.