Does the world really need another book about Tony Hancock? Hasn't every detail of the lad himself's life been dredged up already? Yes and no, respectively. Although the subject of many biographies since his death in 1968, none can truly be called definitive. The first effort, published with some haste in 1969, is coloured by the negativity of co-author Freddie Hancock's volatile and sometimes violent relationship with her ex-husband. Lady Don't Fall Backwards by Joan Le Mesurier, with whom Hancock had an affair towards the end of his life, is also tainted by proximity. Cliff Goodwin's When the Wind Changed is too sensationalist to be of real worth. Only Roger Wilmut's Tony Hancock: Artiste comes close to being a worthy epitaph for Hancock, but, as its title suggests, it concentrates on the work, rather than the man.

John Fisher sets out to tackle both, the main challenge being to work out where the man ends and the work begins. There was a lot of Anthony John Hancock of Birmingham and Bournemouth in Anthony Aloysius St John Hancock of 23 Railway Cuttings, East Cheam. With great skill, writers Ray Galton and Alan Simpson built up the bombastic, bathetic everyman character using elements of the comedian who played him.

Luckily, Fisher is ideally qualified for the job. Since his 1973 book Funny Way to be a Hero, he has written extensively and seriously about comedy, most recently in his 2006 biography of his friend and hero Tommy Cooper. Added to which, his 30 years as a producer and executive in television light entertainment gives him a firm grasp of the artistic temperament, and also the contacts to ensure that everyone who matters has been interviewed.

The only potential problem is that Fisher was nominated as Hancock's biographer by the comedian's brother and one-time manager, Roger. Roger Hancock's formidable reputation as an agent is exceeded only by the ferocity with which he has administered the Hancock estate, ensuring that the comedian's later, less successful shows have remained unrepeated and unreleased. Fortunately, Fisher's work has not been compromised by the connection; indeed, it has been enhanced by access to previously-unmined material including notes made in 1962 by Hancock for an abortive autobiography.

Usually presented as the ultimate sad clown, Fisher shows that the real-life Hancock was, at least until alcoholism took him over, a happy, witty, joyous man with many loyal friends. He was always dogged by anxiety, usually unjustified, about his performances, but off-duty, Hancock was a wild enthusiast and a force of nature.

The years of fame and glory are covered comprehensively, as are the years of the comedian's sad decline. However, Fisher also brings Hancock's early years to life lovingly, from the young boy observing hard-up dowagers at his mother's Bournemouth hotel, storing away the images of faded gentility for future comic use, to the young comedian making his way through RAF service onto the halls. Hancock's stage work tends to be dismissed in favour of his TV and radio oeuvre, but Fisher puts up a spirited defence of Hancock's stand-up, from the informed position of having seen his subject perform in variety three times. The act barely changed over 20 years, but in many ways the deliberately mediocre impressions of Charles Laughton, Robert Newton and George Arliss actually became funnier as they became more anachronistic.

Fisher gives Galton and Simpson their rightful due as writers who truly understood the performer they wrote for. He also notes that some of their scripts presaged Harold Pinter, when the unfair and snobbish estimation of drama over comedy usually leads to the conclusion that Pinter influenced them. Dennis Main Wilson – producer of the first radio Hancock's Half Hours – and Duncan Wood – producer of the television version – are also given full credit for enabling Hancock to give his absolute best and for saying 'no' to him when necessary. Without such sympathetic support (to say nothing of Sid, Hattie, Bill, et al) and discipline, the comedian floundered and never recovered the form of his BBC shows.

One of the traps of biography is to present conjecture as fact. Mercifully, Fisher avoids it, presenting all of the possible versions in each case. Cliff Goodwin's book made much of rumours that Hancock was bisexual. Fisher cites several of Hancock's male friends who received drink-sodden overtures, and tells a story, possibly apocryphal, about a steaming-drunk Hancock performing a lewd act on a sleeping Matt Monro in a railway carriage. However, he balances these with testimonies to Hancock's straightness. Fisher's non-judgmental approach is nowhere more welcome than in laying out the final mystery: why did Hancock take his own life at that moment in time? Was it Joan Le Mesurier's dismissive comment to a doorstepping journalist? Was it first wife Cicely's final refusal to take Hancock back? Was it realisation that he was a pale shadow of his former self? The fact is that no-one really knows, no-one will ever know, and Fisher is clever and honest enough to admit it.

Is this the definitive biography? Yes, it is. Fisher is thorough and unflinching, but also fair and balanced. He tackles the seamier side of Hancock's life, but he doesn't dwell unduly. With copies of this book and Roger Wilmut's, the Hancock connoisseur has everything that needs to be said about this sad but funny, infuriating but delightful man.