Shetland, BBC1, 9pm
PEREZ and Tosh are warned to stay away from Mathias Soderland by the Norwegian Secret Service, who tell them they’re meddling in an operation they don’t understand. With Soderland their prime suspect for Sally McColl’s murder, Perez and Tosh won’t be bullied. However, their investigation is suddenly derailed by a shocking discovery. The cast includes the ever reliable Douglas Henshall, while a fine script by David Kane helps make this unmissable.

100 Years Younger in 21 Days, STV, 9pm
JUNE Brown, Claire King, Roy Walker, Sid Owen, Sherrie Hewson, Shaun Ryder, Russell Grant and Sandra Martin continue with the anti-ageing regime. Nutritionist Thor announces that she is stepping up their cleanse to a liquid diet for the next four days.

Wild Britain, Channel 5, 9pm
A NEW series sets out to explore the hidden world of wildlife, using low-light cameras to capture some of their rarely seen nocturnal behaviour. In the first episode, we get a bird’s eye view of the golden eagle as it hunts its prey in the snowy Cairngorms, as well as an insight into the mating rituals of Scotland’s 400,000 red deer. There’s also a closer look at a bold project to re-tree the Scottish uplands and recreate the wildlife-rich habitat of the ancient Caledonian Forest.

Seven Year Switch, Channel 4, 9.15pm
HOW do you breathe new life into a worn out union? Well, this new series has one possible solution. Four couples, Rachel and Tom, Nikki and Simon, Gemma and Tony, and Michelle and George, try to save their failing relationships by switching partners. They live as husband and wife with the new suitor to see if time spent with another can help solve their problems. Guiding them through the process are relationship therapist Lee Valls and life coach Vena Ramphal.

This Country, BBC1, 11.45pm
THE second series of the mockumentary launched on iPlayer last week, but now it’s getting an airing on terrestrial TV. You can’t blame the BBC for wanting to make the most of it – written by sister-and-brother team Daisy May and Charlie Cooper, it’s one of their best-reviewed sitcoms in years, and the first run was a huge hit online.