The first 45 minutes or so of Trevor Nunn’s production of Lettice and Lovage at the Menier Chocolate Factory Theatre are an utter delight. Peter Shaffer’s three-act play, originally written for Maggie Smith in the late 1980s, begins with a joyous series of guided tours of extravagant but boring, old Fustian House in Wiltshire.

Felicity Kendal radiates with lovable quirkiness as Lettice Douffet, the unsatisfied Fustian House docent (and would-be actress) who takes it upon herself to inject more and more thrilling fictions into her tours to the delight of most of her visitors (and to the horror of anyone who knows better). Her flights of fancy come to an end when Miss Schoen (Maureen Lipman), a trustee of the house, interrupts an invented speech about Queen Elizabeth’s near-death from a fall on the central staircase to demand that Lettice put an end to all this creative storytelling.

The first two face-offs between Lettice, all winning originality (she gives an adorable little hop as she exits after one particularly successful tour) and Miss Schoen, no-nonsense and bureaucratically minded, glow with the sense that this pair, yin and yang, are a perfect match of wits, doomed to disagree on absolutely everything.

The play only loses its edge (and its pace) midway through the second act, once the two have begun to form an unlikely friendship, trading in their barbed asides for gentler conversation pieces. With the tension fully diffused, Shaffer’s writing begins to feel a bit aimless – it’s a pleasure to spend time with these characters, but one misses the titanic battle that kicks off the proceedings. (The rather daffy third act stirs up a bit of that confrontational energy only in the final minutes.)

Luckily, Kendal and Lipman remain enchanting throughout. Kendal’s Lettice is a wacky wonder who insists that she is something special: when Miss Schoen calls her “merely dishonest,” Lettice snaps back, “I cannot accept merely. I do not do anything merely.” Kendal lights up most brightly in the moments when Lettice recalls her childhood in her mother’s French-language touring Shakespeare company (“We only played the history plays”), heartily re-enacting her favourite productions. And notice the delicate physical manifestations of Lettice’s need for affirmation: she frequently pushes her hair back behind her ears in a gesture of self-reassurance that seems to betray an underlying terror of being drawn out of her protective imagination.

Lipman’s vocal and physical comedy especially stands out: Miss Schoen’s crippling fear of cats leads to a monumental series of squeals and gasps as she clings to the banister on the stairs leading to Lettie’s basement flat. Like Kendal, Lipman shows a softer, more vulnerable side, sharing secrets of the past with Lettie.

Even in the moments where Shaffer’s play loses momentum, Nunn’s able-as-always directorial hand pulls things along. Robert Jones’ charming set brings vivid life to a small space and his crucial costumes succeed admirably. (The small ensemble cast rotates amusingly through a succession of wardrobes as the opening scene shifts from one tour group to another.)

Lettice and Lovage may be a bit of a let-down after the brimming first act, but Kendal and Lipman ensure that this particular guided tour is well worth the visit.

Lettice and Lovage is playing at the Menier Chocolate Factory Theatre until 8 July.