Peter Denyer: Actor who played the slow but lovable Dennis in 'Please Sir!

Playing the slow-witted but lovable pupil Dennis Dunstable, complete with a bubblehead of curls, in the television sitcom Please Sir! brought Peter Denyer fame at the age of 21, after just two years as an actor.

The writers, John Esmonde and Bob Larbey, created a set of distinctive characters in Dennis, his fellow 5C class members and the staff at Fenn Street Secondary Modern School. "I looked stupider than anyone else at the auditions," was Denyer's theory on how he landed the role of the classroom dunce.

The programme, which regularly attracted 20 million viewers in a three-channel television age, starred John Alderton as the idealistic, newly qualified teacher Bernard Hedges trying to maintain order among his unruly pupils. The no-hopers ranged in front of him, alongside Dennis, included the wise-cracking rebel Eric Duffy (Peter Cleall), flirtatious Sharon Eversleigh (Penny Spencer, and later Carol Hawkins), the tough-talking "mummy's boy" Frankie Abbott (David Barry), teacher's pet Maureen Bullock (Liz Gebhardt) and the smooth Peter Craven (Malcolm McFee).

"Privet", as the pupils nicknamed Hedges, fared little better in the staff room, where he contended with the weak headmaster Mr Cromwell (Noel Howlett), domineering assistant head Miss Ewell (Joan Sanderson), the philosophical Welsh science teacher Mr Price (Richard Davies), the doddering, old Mr Smith (Erik Chitty) and the former Desert Rat caretaker Potter (Deryck Guyler).

After three series (1968-70), the "pupils" – who already looked like young adults when the programme began – were spun off into the big, wide world and three series of The Fenn Street Gang (1971-73). In his "end of school" episode of Please Sir!, the animal-loving Dennis was still without a job, so Hedges recommended him for a stable lad's position at the local brewery. A previously unseen facet of the gentle schoolboy's character emerged when his bullying, alcoholic father appeared, refusing to let him work there because of a dispute with a pub landlord. Dennis ended up going through a series of jobs in the spin-off series.

Born in Dartford, Kent, in 1947, Denyer was given up for adoption by his mother after his illegitimate birth. He attended Erith Grammar School and acted as a child at Erith Little Theatre. After taking A-levels he trained at the Bristol Old Vic Theatre School, then made his professional début, aged 19 and credited as "3rd Workman", in Alan Cullen's documentary-style musical The Stirrings in Sheffield on Saturday Night (Sheffield Playhouse, 1966), about the rise of 19th-century trade unionism. He also appeared in the 1967 National Theatre production of Peter Terson's football-terraces musical Zigger Zagger in London's West End (Strand Theatre).

After his three-year run in Please Sir!, Denyer and the original cast reprised their characters in a 1971 film version and returned to the television series in a one-off appearance that year, for an Old Fennians reunion. (The programme had continued with a new intake of pupils.) This fame also led the actor to record a single, "Beggar Boy" (1972), although the over-sentimental song failed to break into the charts. Then, in the film Never Too Young to Rock (1975), set in a future when pop music was outlawed, Denyer played the talent scout converting an ice-cream van into a "group detector van" and scouring the country for musical talent – which led him to real-life bands such as Mud, The Rubettes and the Glitter Band during the dying days of glam rock.

The actor had four further regular roles in television sitcoms. He was the cello-playing student George in Moody and Pegg (1974), Michael Simpson, half of a gay couple who were neighbours and friends of the agony aunt played by Maureen Lipman in Agony (1979-81), and the dance-band bass player Malcolm Lockwood, bachelor son of a widower acted by Joss Ackland, in Thicker Than Water (1981). Finally, he was seen as the geeky, greasy-haired Ralph, a member of the 1-2-1 singles club, in the John Sullivan-scripted Dear John (1986-87, plus 1989 Christmas special), which starred Ralph Bates as the husband coming to terms with his wife running off with his former best friend.

Denyer, who briefly played the NY Estates worker Batty in the soap Emmerdale Farm (1979), also wrote an episode of the caravan-park sitcom Romany Jones (1973), as well as stage adaptations of A Christmas Carol (Piccadilly and Victoria Palace Theatres) and Peter Pan (Yvonne Arnaud Theatre, Guildford, and Chichester Festival Theatre). Eventually, the shy actor gave up performing in favour of writing pantomimes. He moved to Cheltenham in 1978 and, five years later, helped to secure funding from the borough council for two professional shows, Dick Whittington and Aladdin, staged at the town hall in an attempt to keep audiences there during the temporary closure of the Everyman Theatre. Denyer wrote and directed both productions, as well as playing the Dame in Dick Whittington.

Then, in 1986, he teamed up with the producer Kevin Wood to write, direct and produce pantomimes across the country, which he continued to do until his death. Denyer – who never married – claimed to be Britain's most performed playwright of recent years, with more than 200 productions each year using his scripts.

Peter John Denyer, actor, writer, producer and director: born Dartford, Kent 19 August 1947; died Cheltenham, Gloucestershire 18 September 2009.