Twenty-one years ago this week a sitcom arrived on British television involving three characters so improbable that they held the nation in thrall. It had started as a French and Saunders comedy sketch about a hedonistic ‘modern’ mother (Eddy) and her appalled, straight-laced daughter (Saffy). To spin this out into a series, Jennifer Saunders added Joanna Lumley as a hard-nosed, hard-drinking best friend (Patsy) and two essential props: Bollinger champagne (Bolly) and Stolichnaya vodka (Stolly).