The closing credits show Kirsty Dillon playing the part of "WDC Stephens". In this episode, "WPC Stephens" (per DCI Barnaby's own words) is promoted to Detective Constable (or "DC" without a "W", again in Barnaby's own words). There is no rank of "Woman Detective Constable" and by this time, even the use of WPC has been deprecated in the UK, replaced simply by "PC" regardless of gender. So while it's factually incorrect to use "WPC" and "WDC" in the real world, that's not how things are done in the reactionary county of Midsomer. An inside joke to be sure.