Call the Midwife, Series 3, BBC One

If it ain't broke don't fix it, and writer Heidi Thomas obviously has no intention of tinkering with the Call the Midwife formula. Virtually nothing has changed, except that there's a new character, Sister Winifred, while Chummy (Miranda Hart) is now living with her husband PC Noakes (Ben Caplan) and has a baby son. However, you can't keep a born midwife down, and Chummy's return to the Nonnatus House mothership by the end of the episode was a foregone conclusion.

To be fair, the scenery has been altered slightly, because the nuns and midwives have moved out of the old Nonnatus House into a former seamen's mission (complete with now uncalled-for urinals), but since it's just another batch of Fifties-style props on a set you'd hardly call this a game-changer. Meanwhile the working-class women of Poplar are still prone to going into blood-curdling throes of childbirth at a few seconds' notice, poverty and ignorance remain implacable enemies, and Vanessa Redgrave still does those cringe-making bits of preachy voice-over ostensibly from original author Jennifer Worth: "...always a sense of life forging forward, pulsing like the river Thames itself..." But perhaps those are now Thomas's handiwork, since the new episodes of Midwife have been created from scratch now that Worth's books have been used up (Victoria Yeates as Sister Winifred, pictured below).

The appeal of the series, other than to connoisseurs of shrieking child delivery in various insalubrious locations, lies in its gentle observations of character and of the grimly primitive social conditions of 60 years ago. Jenny Agutter remains brisk and matron-like as Sister Julienne, Pam Ferris lends some welcome Sergeant-Majordom to Sister Evangelina (you could imagine her sending her nuns out on cross-country exercises with live ammunition), and Judy Parfitt's Sister Monica Joan floats through it all on a magic carpet of euphoric lunacy. This week, her obsession with rearranging her much-treasured library paid dividends when she was able to assist Dr Turner (Stephen McGann) with an 18th-century text about infant diseases, which led to the revelation that he had encountered one of the first known cases of cystic fibrosis.

In fact, you could sometimes be forgiven for imagining that you were watching a public information film from NHS Direct, as the midwives delivered bulletins about preventing infection and the paraphernalia required for giving enemas, while there was a suggestion that Monica Joan's unusually febrile state may have been the result of a urinary infection. It's lucky they've kept Miranda Hart on board, because without her ditzy but determined performance as Chummy, her dialogue peppered with archaic posh-gel slang ("bally botheration! ... one more ballynormous push!"), Call the Midwife would be a wintry patch of Sunday evening indeed.

Chummy, neurotically incapable of just sitting around attending to child-minding chores, outdid herself by organising a gala opening celebration for the new ante-natal clinic. The cherry on the cake was a visit from Princess Margaret (pictured left), the royal personage shown only from behind as she glided beatifically into the room. Thank heavens HRH wasn't caught short by a sudden breech birth crisis. 

• Call the Midwife Christmas 2016 Special reviewed on theartsdesk