Lynda Bellingham played many roles during her five-decade professional career, but became synonymous with one. "Being a mum making gravy was not quite how I had seen my career advancing," she said once. But between 1983 and 1999 that's what she did in 42 "episodes" of an award-winning TV ad. Since the early 1980s, her name was rarely mentioned in print without it being prefaced with "Oxo mum". During her career, though, she starred on TV as the vet's wife Helen Herriot in All Creatures Great and Small in the 80s and as one of two divorcees tr…