Dawn French is in a cottage in sodden Wiltshire, where she’s filming Lark Rise to Candleford, adapted as a drama series for BBC1. She’s playing a pregnant woman. She has to wear a corset. Her hair is a little redder than its usual chocolate because she didn’t want to wear a wig for the part. She has pink cheeks, no make-up, and no lines, either. “Imagine, why am I supposed to be pregnant at my age? I’m 50.” She’s only 49, 50 in October. Typical French. More comfortable with the bleaker view. Yet she is endlessly chirpy, frothy, friendly.

It’s a neat, sweet cottage. She took it on so she wouldn’t be tempted to party with the rest of the cast, who are in a hotel, and she wanted to write her memoirs. Reflection is the mood. After more than 20 years of being on TV together, the double act French and Saunders are putting together a retrospective – A Bucket o’ French & Saunders. Although it is hung together around new material that is still infected with the earlier savagery and outrageousness, this probably is the end of French and Saunders as we know it. Over the years we’ve watched them change and grow. The chemistry between them is still compelling. Saunders, deadpan; French, ridiculous.

They met at the Central School of Speech and Drama in 1977. Didn’t like each other to begin with, but ended up sharing a house. They’ve had highly successful separate careers, but even after Absolutely Fabulous and The Vicar of Dibley, there’s a comfort in thinking of them together.

French lays out chocolate crispy cakes and chocolate brazils and makes coffee. She also lays out photos from her album spanning 30 years. Hauntingly young faces of her husband, Lenny Henry, Saunders, Alexei Sayle, Rik Mayall and Saunders’s husband, Adrian Edmondson. The Comic Strip years, the Young Ones years. Everyone much thinner but with terrible hair.

In front of me is a lifetime in comedy and romance: Saunders looking thoughtful in her wedding dress, being squeezed tightly by French in a pink pillbox hat. Their first appearance on stage together at Central involving French in a corset and Saunders posing with a feather duster. Before all the extravagance of the costumes, there are pictures of them in an early stage show, where their expressions alone are the comedy – Saunders’s curling lip and French’s over-enthusiasm. French says: “Look, she’s even got her heels on with a tracksuit. She wears heels a lot. I don’t. I’ve never been particularly safe on heels. In fact, she and her youngest daughter, who’s 16, swap shoes quite a lot. Jen’s got great legs, so she can get away with those wraparound dresses, you know, Diane whatsherface [von Furstenberg], and she looks quite ‘sexy office worker’.”

French says all this with a mixture of pride, disbelief and affection. She is the least likely person to swap shoes with her daughter, Billie, who is 15. French’s shoes, lined up in a corner, are flat, colourful, sensible. She shows me a picture where she’s blonde and tousled. “I was going a little bit Farrah Fawcett. Look at those terrible highlights.” She actually looks glamorous, but French tucks the photo away and finds another – her favourite. They are sitting back to back, dreamily gazing away from each other. It was taken at the time of the Comic Strip TV show. They’re gazing into a future that is now a past.

“We are almost polar opposites to what we used to be. She was incredibly disorganised and messy.” Even now, Saunders pulls off a dishevelled air, yet she has written three TV series in less than a year – Bucket, The Life and Times of Vivienne Vyle, and Jam and Jerusalem. French lights up, though, when she describes her friend’s old messy ways.

“When we lived together in Chalk Farm she had a room at the top of the house. We got broken into and the police said, ‘Well, it is quite bad, but the worst is that room at the top.’ And, of course, nobody had been in there. She used to be up to her knees in old pants. Something changed when she had the kids. She got organised. Now she can’t stop sweeping.” Indeed, Jennifer had already told me that sweeping helps her think.

They have committed to going on a farewell tour next spring. French talks about Saunders with palpable love. It was not, however, love at first sight. Saunders immediately disliked French because she actually wanted to become a teacher (they were both doing a B Ed in drama and spoken language) and she seemed bubbly and confident, whereas Saunders loathed the idea of becoming a teacher and hadn’t quite grasped that was what the course was all about. Both had fathers who had been in the RAF, though Saunders came from an upper-crust family. Her mother was disheartened when her daughter couldn’t engage with the idea of going to Oxbridge. In fact, Saunders couldn’t engage with much. She came over as lazy, but she was fearful of failure, so she put things off. She told me: “Dawn was the motivator.” And French agrees that’s the way it used to be. “The good thing is that we are very different. In some ways I’m more confident. I am the anchor for certain things, she is for others. She’s much more writerly than me.”

Ab Fab started off as a 14-minute sketch in a French and Saunders show. They had time booked in the studio for a new series but suddenly an adoption agency called and said they had a baby for French. Her need to nurture was primal and vast. Looking after a new baby, she was worn out, so Saunders took the studio and did Ab Fab herself. Thus French was not threatened or jealous of its success. She was proud, and happy to be at home with her family. “I got very ‘cardigan’. I loved mashing carrots. But I couldn’t think any more.” This was a shift in their relationship. Saunders has had three children: Ella, 21, Beattie, 20, and Freya, 16, working through her pregnancies. French had always been the doer, Saunders the thinker. Then Saunders became the thinker and the doer.

“She’s like a shark in the water, Jen. One of those people who’s always swimming, always thinking. When she smells the blood, whoosh, the eyes close over. She goes into a totally concentrated thing; nothing fazes her, nothing frightens her. She can procrastinate for ever – she’s very skilled at that. But then she goes for it on the deadline. She once had Concorde wait for a script… ” Now it is French who is afraid of writing on her own. “I’m writing my memoirs, and it’s really hard. I’ve never had the bizarre experience of doubting all the time. I thought I’d look forward to it, as I like my own company…”

French is in equal parts vulnerable, open, accessible and covert. In some ways she confronts her “warts”. She is more comfortable being insulted about how she looks in these photographs than being complimented. Part of her motivation for writing a book is that someone wrote a biography without even meeting her. “It was a sort of rape. It lied about my mum and dad’s marriage, saying they split up when they never did. My mum still cries about it.”

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