It has been a tough few weeks for Colin Dexter, with his heart and eye conditions necessitating visits to the doctor and a short spell in hospital. So, it came as something of a surprise to find him fresh, not from the John Radcliffe, but an altogether different kind of institution near Bicester.

“Has he told you? He’s just got back from prison,” said his wife, Dorothy, with a glance towards her husband, who though still far from well, had ended up getting soaked during the course of his visit.

No doubt the man who has been responsible for more than 80 fictional murders in Oxford left behind 300 real-life criminals with more optimistic outlooks, if not the key to committing the perfect homicide.

But not all of the prisoners may have realised just how lucky they were to be addressed by the creator of Inspector Morse. As he nears his 80th birthday, Dexter is still in huge demand for speaking engagements, with the world’s fascination with Morse and Lewis showing no sign of fading, even if it is a decade since he killed off the cerebral inspector in the last Morse novel The Remorseful Day.

He now reluctantly is unable to accept many of the invitations he receives and recently even had to pull out of a talk.

“It is the first time I have ever had to cancel any engagement. But there it is,” he said, as mortified as Morse having to buy a round of drinks.

In a way, the continued interest is hardly surprising. Rare is the week when you cannot catch at least one episode of either Lewis or Inspector Morse on one of the ITV channels. But Dexter never watches them.

It is not that he remembers the stories too well. Quite the reverse, in fact. His deafness means he simply is unable to follow what is going on. But if loss of hearing is a frustration that he has had to put up with for many years, it is his deteriorating vision that most saddens him.

“You know how many books I’ve been able to read in the last year?” he asks. “Just one. It was a very good book though, The Last Days of the Romanovs.”

It has also meant a painful withdrawal from his 60-year addiction to cryptic crosswords, a habit which he passed on to Morse, of course, along with a taste for good beer and a love of Wagner.

“I used to think the day started with getting a paper and sitting down and doing a crossword, some days for longer than others. It was the same with Morse, who’d say, ‘don’t interrupt me now, Lewis — I was close to finishing until you disturbed my concentration’. Morse couldn’t start a new case until he had finished.”

But you simply cannot keep a good crossword obsessive like Dexter down, and if he can no longer get his daily fix of anagrams and cryptic definitions each morning, he has set himself the task of turning the rest of us into happy solvers.

Still need a clue? Well, Colin Dexter has just published Cracking Cryptic Crosswords — a personal guide on how to cope with cryptic crosswords, allowing those humble souls who have limited themselves to quickies to finally see the light and savour the altogether more satisfying delights of the cryptics.

“Well, it’s only a slim volume, and a few friends have encouraged me to write it before my light goes out forever,” he points out. “In addition to the real possibility this pastime offers of escaping the anxieties of Alzheimer’s, it is the most serene and civilised way of wasting time that I, now nearing 80, have as yet discovered,” he tells his readers.

The friend who was most instrumental in getting him to share wisdom accumulated since his early teens — when he was confronted by a school pal’s parent with “nothing squared is cubed” — is Jackie Gray.

Mrs Gray, the former city councillor and now a successful publisher, watched enthralled, when she chanced across a preoccupied Dexter on an Oxford to London train.

“I watched him complete The Times cryptic crossword in the 15 minutes it took to reach Didcot,” she recalls. “As someone who had never been able to progress from the ‘easy’ crossword to the cryptic variety, I was in awe. Colin assured me that I could do it, too, and then proceeded to tell me how.

“He reverted to the schoolmaster that he once was, patiently translating the clues into a language I could understand. My gratifying reward was that for the first time I was able to fill some answers into a cryptic crossword grid having actually interpreted the clues.”

The idea of the book was born. It has been many years in the making, with Dexter even using the idea of a ‘Didcot challenge’ in a Morse story. As with the women in the inspector’s life, its was a case of so near and yet so far, with Morse, unlike his creator, defeated by just two clues.

Dexter certainly had no crossword teacher in the family home in Lincolnshire, where he grew up.

“Both my parents left school at the age of 12. My father was a taxi driver. Neither of them had any secondary education at all. They said that they never had a chance and impressed upon us that we as children still had a chance and must make the most of it. So, we never had to do work around the house, doing the washing-up or tidying our rooms. What was expected of us was to sit down at the kitchen tables, open our books and try as hard as we could.”

When I ask whether he had been able to turn Morse actor John Thaw into a crossword enthusiast, he shakes his head sadly. For Thaw did not receive such parental encouragement. Abandoned by his mother at the age of seven, Thaw went to a school in Manchester’s tough Moss Side area, passing only one O-level. The man who played the cultured Oxford-educated detective so wonderfully, always remained acutely conscious of his limited education, even failing to acknowledge his own brilliance as an actor.

Dexter recounted their conversation a few days before Thaw died of cancer. “He said, ‘I can never understand why I’m thought of as such a good actor — what I can do much better than others is learn my lines’. It was such a wonderful self-effacing view of his own talent. I loved him for that.”

Education has remained one of Dexter’s great passions throughout his life. For all his fame as, arguably, Britain’s best known living crime writer, he insists he would have been far happier to have remained a Latin and Greek teacher, his chosen profession, which only the onset of deafness forced him to leave to join Oxford University’s Schools Examination Board in 1966, where he supervised O, A, and S level exams in classics and English, until his retirement in 1988.

He was to get his name in print thanks to Robert Maxwell, for whom he co-authored a number of textbooks. “He was a charming man. A charming crook,” recalls Dexter.

He only began writing detective stories in 1973. But the work that gave him most pleasure was setting the cryptic crosswords in The Oxford Times.

It was a challenge he met for 15 years, sharing the compiling with Bill Hudson, a former editor, and John Chavasse. The competition to come up with the best clues was fierce and a grin crosses the writer’s face as he recalls their efforts to outdo each other at regular meetings. So, in a sense his earliest fans may have little need for his new book, at least those cryptic fiends who would not flinch from “Naughty did many a service” or “Perplexed damsel wanting a head and requiring nothing more”.

But I suspect that only the very best will not find some new secrets to help unlock the cryptics. For the cryptic virgins it will be a godsend, with Dexter showing how it is possible to identify the different types of clue, with chapters on anagrams, hiddens, abbreviations, ‘aliens and dots’ and not forgetting, of course, ‘homophones and puns’.

But there are added inducements. He lists his all-time favourite 30 clues (his the top three being “We’ll get excited with ring seats”; “In autumn we’re piling up the last of the leaves”; and “Marlborough’s second crusher in conclusive quartet of victories”) and, best of all, a Morse story, albeit a very short one.

“You know all the books are still in print,” he says. “The books are all right and the character is all right, but people forget that Oxford is the big character. People say, ‘you were at Cambridge University, why didn’t you set it in Cambridge?’. But I was only there for four years. If I had not come to work in Oxford and been able to walk around the streets and go into the pubs, I just wouldn’t have been in the right situation to write the Morse books. I think luck plays a bigger part in life than many people are prepared to admit.”

In front of him lies The Times crossword untouched. But after so much killing, beer drinking and pleasure given to tens of millions, it is fitting that he has completed one more book that neatly brings together his two great loves — teaching and crosswords.

“Yes, before the light goes out,” he says, before quickly adding, “Did you know that one of the meanings of ‘light’ is ‘clue’?”