The Real Marigold Hotel (BBC One, 9pm)

Making TV programmes isn't easy.

Everyone wants to create a hit, but how do you do that? Some producers decide to copy a previous show's success, making it just different enough to avoid any trouble. You could perhaps describe them as a homage instead.

The brains behind The Real Marigold Hotel did something very smart indeed. They took the premise of the film The Best Exotic Marigold Hotel (which was based on the book These Foolish Things by Deborah Moggach) and tried to find out if British pensioners really could make their pensions stretch further by moving to India.

"BBC Two has a strong track record of creating bold new factual formats and Marigold proved to be an instant break through," says the channel's editor, Patrick Holland. "Who would have thought that a show about retirement would become such a hit with audiences young and old?"

Well, perhaps it's down to the fact that we are, according to statistics, an ageing nation, so the subject matter appeals to more of us - we're either around the same age as the participants or, if we're lucky, we will be one day and want to know want options lay ahead.

The first series saw the celebrities spend three weeks sampling the sights and sounds of Jaipur, capital of the state of Rajasthan in Northern India. They set up home in a haveli - a private mansion - where they had to work out whether they could take care of themselves or need staff to lend a hand.

That run proved to be so popular that the second series moved to BBC One, with a new batch of famous faces setting up home in Kochi; two spin-offs saw celebs from both seasons travelling to Florida, Kyoto, China, Cuba, Iceland and Thailand.

Now a third series is upon us.

"We've got a brilliant line up of famous faces checking into The Real Marigold Hotel," says David Clews, director of production company Twofour. "They're a fabulous and funny mix of characters, honest and open about their personal stories, and all of them excited to explore what retirement has to offer in the vibrant and beautiful city of Udaipur.

"We've got our first married couple too, which adds a brand new dynamic. It's going to be the best series yet."

One of the joys of the programme is seeing new friendships form between the participants - and they're certainly a mixed bag this time around.

It's hard to imagine what former newsreader Selina Scott and actors Stephanie Beacham and Susan George will find to say to Ian and Janette Tough, aka The Krankies; they simply come from very different worlds.

But the Toughs will, you would think, bond with fellow comedian Syd Little. He also moves into their idyllic new home on the shores of Lake Pichola alongside ex-EastEnder Peter Dean, former jockey Bob Champion and author Stanley 'father of Boris' Johnson, who knows how celebrity-driven shows work having spent time in I'm a Celebrity... Get Me Out of Here!

Here's hoping his stay in India is rather more relaxing.

Animals Behaving Badly (BBC1, 8pm)

In the final episode, Liz Bonnin meets the animals going to extreme measures to survive, and reveals the some of the surprising scientific discoveries behind their ingenious behaviour. Liz examines the behaviour of cockatoos vandalising houses in Sydney, chimpanzees using bullying tactics to get to the top, crabs who hold anemones hostage to protect themselves, and the stone martens who cause millions of pounds worth of damage to cars while protecting their territories. Last in the series.

Picnic at Hanging Rock (BBC2, 9pm)

A series of flashbacks tell the story of the missing girls in the months leading up to the picnic. Meanwhile, Hester struggles to contain mounting hysteria in the school, as students are withdrawn from the college by worried parents. She takes the remaining girls to church as a display of normality, but leaves the young orphan Sara behind, as punishment for ongoing defiance. A six-part reimagining of Joan Lindsay's 1967 mystery novel, starring Natalie Dormer, Samara Weaving, Madeleine Madden and Lily Sullivan.

The Bletchley Circle: San Francisco (ITV, 9pm)

It becomes clear that San Francisco is in the grip of a killer with a dark and complex agenda now menacing Iris's own neighbourhood. As the four unlikely allies struggled to be able to work together, they not only expose a twisted operation, but also forge powerful and lasting bonds. Drama set during the social change of the mid-1950s, following four women gifted with extraordinary intelligence, capacity for pattern recognition and decryption. Starring Rachael Stirling, Julie Graham, Crystal Balint and Chanelle Peloso.