MY first question to Paul Whitehouse and Bob Mortimer does not go entirely to plan. We've me to discuss their new book Gone Fishing, based on their BBC show of the same name: What led them to think that their regular fishing trips might play well on BBC Two?

"I'm not sure I remember," says Mortimer.

"I do," says Whitehouse. "Were we drunk? We'd seen a very big fish under a tree, and my cast had hit the tree and dropped in, and I gave you the rod..."

"And I f***ed it up," Mortimer chips in. "It was all so quick. Tree, boom, woah! We were probably doing daft voices, and saying something a bit politically wrong."

"Oh yes," says Whitehouse, pausing to politely request extra sugar for his tea. "I think the closest thing we associate it with is the Detectorists, and Tim and Pru - that sort of British, rural show."

A few more non-sequiturs later and I'm none the wiser as to how they came to pitch the show, but I might have hit on why the BBC commissioned it. Their interplay is of the sort reserved for old friends, and the fact they've spent much of their careers in double acts - albeit not with each other - is clear to see.

The book is as breezy as their banter, but, fortunately, a little more succinct in its explanations.

Gone Fishing actually began not on the riverbank or in the recording studio, but on the operating table. In 2015, Mortimer had a just-as-scary-as-it-sounds triple heart bypass, having been warned by his doctor that without surgery, he wasn't likely to make it through his upcoming tour. The procedure revealed that 95 per cent of his arteries were blocked.

To aid his rehabilitation, Whitehouse – himself the recipient of arterial stents five years previously – cajoled his pal out of the house, promising to teach him to fish. It was a long-held plan that never quite got off the ground – they had attempted to fish together once before, some years earlier in London's Green Park.

For Whitehouse, 61, fishing brought back memories of his childhood – hooking trout with his father on the banks of the River Lea – but for Mortimer, 60, it was a brave new world at a time when just moving made him nervous. "I was very scared after the operation," he admits. "When Paul was trying to get me fishing, I felt safer sitting by the fire. That's the story of the book really – that I didn't do that."

Their riverside chats spawned the show ("We think we're f***ing hilarious," says Mortimer), and the book followed soon after. Both strike a similar tone: laugh-out-loud funny, cerebral, nostalgic, and occasionally bittersweet. It's simple humour on complex subjects, delivered by two men that know they're starting to get on a bit.

After a long day of sitting in deckchairs on the riverbank, their musings extend to the apres-fish. "We always enjoy the evening," says Mortimer, "having a meal and a pint. These days a lot of pints feel like obligation pints, but after you've been on the river all day – that's different."

I'm surprised they're still allowed to drink, after their health scares. "Standard doctrine is obviously don't drink a lot," says Paul, "but I've never been told not to drink anything at all."

About the dinner table, their doctors have been more prescriptive. "Trying to give up cheese, trying to give up meat, trying to give up butter - it feels major," says Mortimer. "You don't realise how much fat is in a cake or a croissant."

"Croissant is the killer," agrees Whitehouse. "I saw it on a list the other day. Bad things: Top – croissant."

As is often the case following a brush with mortality, the pair have gained a penchant for new horizons. "After 30 years, another sketch show just isn't as exciting," says Mortimer.

Whitehouse chimes: "You start thinking, 'Do I really have to put on that bloody prosthetic nose?'"

Mortimer in particular began embracing different genres, and started a football podcast with Andy Dawson called Athletico Mince. "I thought: 'Time's running out'," he says, "so I did almost make a decision. I'd always wanted to do something about football, so I did the podcast."

Never previously big on panel shows, he also made a series of scene-stealing appearances on Would I Lie To You?, which, he admits, might now be what he's best known for.

Gone Fishing marks their most adventurous project to date, and has now brought an equally adventurous foray into authorship. "I think the reason it's done well is that it comes from a genuine place," says Whitehouse. "It's not a contrivance – it literally came from being on a riverbank as a result of heart rehabilitation."

He reveals that, although humour followed as a natural consequence, it wasn't originally pitched or commissioned as comedy. "We spoke to the factual department – we didn't even speak to the comedy department," Whitehouse explains.

As for the book, Mortimer insisted there was a chapter on fishing technique – 36 pages are devoted to baiting, hooking and the rest. The pair admit it's a tough art to teach from the surface of a page though.

"All you can really do is inspire the imagination," says Whitehouse. "And in that respect, I think our stories might be more inspiration than me explaining how to attach a float."

Amid the childhood memories, medical misfortunes, and potentially ill-advised pints, it can be easy to forget that Gone Fishing is still, at its heart, about fishing. But throw years of friendship and humour into the mix, and the result is so much more.

Mortimer & Whitehouse: Gone Fishing (Blink Publishing, £18.99)