Only the British, quite possibly only the BBC, could ever try to produce a television sitcom based on a bunch of old codgers, with barely a functioning limb between them, up for defending their little bit of the country from the opposing Nazis just a few miles across the channel - armed only with one gun and some broom handles. David Croft and Jimmy Perry are, not for the first time, divinely inspired with this charming comedy that puts Arthur Lowe "Capt. Mainwaring" (pronounced Mannering), the town's pompous bank manager in charge of a platoon that features his clerk, the rather weedy but…