Who is Mrs. Gates from The Nine Tailors: Changes Rung on an Old Theme in Two Short Touches and Two Full Peals and what is their importance?

Mrs. Gates is the housekeeper at the Red House. She is snobbish and class-conscious, but this behavior allows the police to work out when the body was put into the grave - Mrs. Gates noticed when her simple and "appropriate" wreath for Lady Thorpe's grave was moved aside, contrary to her very strict instructions.