The Guardian described Letts on his death as "a pioneer of British television" who "served the medium for more than half a century" and "secured his place in TV history" with Doctor Who.

Letts was an assistant stage manager at Leicester's Theatre Royal in his teens and took up the job full-time after leaving Wyggeston Grammar School for Boys.

[4] This was a complex serial to direct as Troughton played both the Doctor and the dictator "Salamander" in the same story and sometimes in the same scenes – a rare and demanding directorial requirement for the 1960s.

It was an era of substantial change for Doctor Who, with episodes broadcast in colour for the first time and an improved budget which enabled more location filming and action sequences than had previously been possible.

This was a profoundly significant change: it allowed much more rehearsal time, in a much less frantic atmosphere; it ended a long running dispute with the unions representing the technical crews; it reduced wear-and-tear on the sets (and the budget allocations for repairing the damage); and it meant that, forever after, serials could only be made in multiples of 2 episodes—a primary reason for the 4-episode and 6-episode format dominating the schedules for the following fifteen years.

Letts formed a particularly close partnership with two other contributors to the programme: Terrance Dicks, who was the script editor on the programme between 1968 and 1974; and playwright Robert Sloman, with whom Letts co-wrote four serials in the Pertwee era: The Dæmons (credited under the pen-name Guy Leopold); The Time Monster; The Green Death; and Planet of the Spiders, which was Pertwee's swansong.

According to Toby Hadoke, who contributed to his Guardian obituary, "Letts's liberal worldview led him to commission stories with contemporary resonance – eco-parables, critiques on colonialism and apartheid, even entry into the Common Market (the Galactic Federation in Doctor Who parlance) were all presented within a format of child-friendly derring-do.

When the programme returned in 2005, Letts was involved in the hectic round of interviews to promote the show, appearing for a lengthy discussion piece on The Daily Politics with Andrew Neil on BBC2.

Letts also wrote the scripts for two radio plays based on the show, starring Jon Pertwee with Nicholas Courtney as the Brigadier and Elisabeth Sladen as Sarah Jane, broadcast in the 1990s: The Paradise of Death and The Ghosts of N-Space.

He also wrote two original Doctor Who novels published by BBC Books: Deadly Reunion (co-written with Terrance Dicks, 2003)[9] and Island of Death (2005).

He, like Terrance Dicks, also wrote radio dramas for the Big Finish company's series of productions starring Elisabeth Sladen as Sarah Jane Smith, released on CD.

He oversaw more than 25 serials in this capacity, over an 8-year period,[10] including Nicholas Nickleby, Great Expectations, A Tale of Two Cities, Dombey and Son, The Hound of the Baskervilles (starring Tom Baker), The Invisible Man, Pinocchio, Gulliver in Lilliput, Alice in Wonderland, Lorna Doone, Little Lord Fauntleroy, The Children of the New Forest, Beau Geste and Sense and Sensibility.

Many actors with whom he had worked on Doctor Who were to feature in these classic serials, including Tom Baker,[11] Elisabeth Sladen,[12] Caroline John,[11] and Paul Darrow.

It had been intended for Letts to attend the Doctor Who Appreciation Society's convention 'Time and Again' at Riverside Studios in Hammersmith that year, until it became clear his health would not allow this.