Review from the Radio Times website:
"This two-part doomy, gloomy, not-so-festive special is quite odd. It appears to be set not in 2021, but in December 2020, back when the tier system was about to go haywire and our Christmasses would all be ruined.Yet there’s no plot-specific reason why Casualty has opted to rewind the clock, as the main storyline, which involves the reappearance of Holly Aird as Munchausen by proxy sufferer Laura Merriman, could easily have taken place in the present day. And with on-screen captions not exactly making matters clear, I expect many will be left puzzled by such sights as Iain back working for the helicopter emergency medical service and Charlie grieving so acutely again for Duffy. Rather than being thrilling, it’s borderline baffling."
Yup, it's Aunty Beeb's idea of a Christmas present to fans of the series. Perhaps they'd rather have socks, though?