Some of the greatest hits of the Sixties usher us into the auditorium, and the simple set – a bachelor pad in New York – is also a mirror-perfect reflection of this time. Black-and-white pictures of Judy Garland and Bette Davis hang down, showcasing some of the icons of the era. And these are, of course, gay icons too … and so the stage is set for the revival of this celebrated play, often credited with transforming attitudes on its 1968 debut.

A birthday celebration is about to begin, and the start of the play whips along merrily, as Michael (Ian Hallard) and his Saturday-night buddy Donald (Daniel Boys) prepare drinks and canapes. It’s in honour of Harold (Mark Gatiss), the birthday boy – but as the other guests arrive, there is an unexpected and unwanted surprise amongst them. Cue tight-lipped Alan (John Hopkins), Michael's room-mate from college way back when.

The first half of the action light and humorous, full of great one-liners, and the actors successfully create the impression that they are genuinely having a party together – with we, the audience, voyeurs on what unfolds. The mood is compelling and very easy to get caught up in, not least because of a lovely, simple dance routine that made me wish I could join in. The nine men between them build a totally plausible and absorbing friendship group, revealing both the alliances and the cracks in the varying relationships. James Holmes deserves special mention as the ageing Emory, delivering an eye-catching performance wonderfully embodying the phrase “gay abandon”.

But the arrival of Gatiss’s Harold changes the whole mood and atmosphere. Gatiss is superb as the acidic, ironic “Jew Fairy”, delivering cutting lines which evoked laugh-out-loud responses from the audience. Things get even darker when a game of declarations – “Affairs of the Heart” – comes into play. It does what it says on the tin, and Gatiss and Hallard stack up the tension gradually between them, as Hallard’s Michael transforms from the charming host to a visibly spiteful and terrified wreck.

We've all got stories from parties which turned sour, without our quite knowing how; this is a bit like that. But the stumbling block for me was the ending, which feels far too sudden and abrupt. The sense of “Huh?” from the audience members around me was palpable, almost audible; something else was needed to dispel the anti-climax of what, up to then, had been a great performance.

Nevertheless, this production – superbly directed by Adam Penford – offers an important insight into the secret shame and occasional self-loathing associated with love which, at that time, “dared not speak its name”. Witnessing Michael’s torture as he tries to live a double life, I could only feel gratitude towards the courageous trail-blazers who led the way in changing attitudes over the last forty years.  Playwright Mart Crowley is among them, and I think he’d have been happy to see the out-and-proud audience around me at the Theatre Royal.  All in all, a treat of a night.