I n a north London TV studio, there’s a sense of unpredictability in the air. A gaggle of singing teenagers are on set; there’s a dog traipsing around; and – just down the hall in the canteen – Joanna Lumley has paused our interview to very politely ask a catering lady not to pack up her tangerine for her. “Darling, I literally cry with gratitude but I don’t need it in a box this time, it can travel on its own,” she purrs. She’s as poised as you might imagine – even if she looks ready for an arctic expedition, wrapped in a big mustard puffer jacket against the December cold. “Sorry, I’ve gone off on a tangent.” We’re talking about Amandaland, the funniest and biggest comedy on British TV.…