So was the great director Michael Blakemore, who acted with the company for a couple of seasons (busy completing what sounds like an explosive, dirt-dishing memoir about his time at the National Theatre as it passed from Olivier to Peter Hall), and Toni Kanal, widow of the critic and jazzman Benny Green, a great friend of Conville and the theatre.
It was one of those tears and laughter occasions, with speeches on the picnic lawn and lunch in the Robert Atkins studio. The sun shone and white clouds scuttled across the tall trees. Time stood still and champagne flowed. On my way in I was telling Richard Digby Day that my first visit to the park was to see David Buck as Cyrano de Bergerac. But who was the Roxanne? It should have been somebody like Gabrielle Drake.
"It WAS Gabrielle Drake," yelped Digby Day. "I should know. I directed the show, and it was my first job here." With the current regime of chief executive William Village (who hosted the party) and artistic director Timothy Sheader flying so high - they have won the Olivier award for best musical revival three years running - it's easy to forget that the place was falling apart at the end of the 1950s before Conville came along and picked up the lease.
In 1962, he formed the New Shakespeare Company and appointed a very fine actor and Shakespearean, David William, as his resident director. On the opening night, 4 June, the weather was abysmal. Milton Shulman sat in his deck chair - there were no "seats" in those days - which promptly collapsed under him; still he gave the show a good review, said Conville. Shulman, critic on the Evening Standard, was a large man but very fit - he was a keen and competent tennis player and a member of Queen's Club - so he took the tumble in his stride.
Conville stayed at the helm until 1986, appointing Digby Day after William, and then Ian Talbot, who gave his Bottom in the park no less than eleven times. During that period, said Digby Day, Biggins was an "improbable" Puck - "I was the best Puck in the park that year," interjected the cheery chubby chap, slightly miffed - and the Queen and her consort paid a visit in 1982, the year of the theatre's golden jubilee.
Digby Day remembers nothing of what the Queen said when she met the company backstage apart from one enthusiastic aside: "Oh, Philip," she cried, "do come and look at Puck's boots!"
Ian Talbot acted in the park from 1971 onwards. Conville got the new auditorium built in 1975 at a cost of £150,000, and followed that with a workshop, a new box office, kitchen and the picnic lawn. As Conville retires, the theatre has completed a £3.3million redevelopment of the backstage, box office and studio, financed 82% from theatre reserves, 6% from the Royal Parks and 12% from fund-raising.
It's an amazing story, and it's all happened with a steady accumulation of talent and invention in the production styles. Talbot recalled that they shivered in their boots a bit when Shakespeare's Globe opened in 1997. But this was a false alarm. Their figures went up. The experience in the park is totally different from that in the Globe, and the audiences do not much overlap.
And Robert Davis of Westminster City Council, chairman since 2009, paid fulsome tribute to Conville's vision, determination and tenacity. Like most people who work there, and most people who go there, he simply loves being in the place. Even when it rains.
Conville was delighted that his guest list included a Hamlet, a Holofernes, three Toby Belches and several Titanias. His designers were well represented, too, by Peter Rice and Tim Goodchild, and there were fond memories of his late wife, Philippa Gail, a famous and feisty Hermia, who died of cancer aged 56 in 1999.
Talbot had stories of Conville, whom he regards as a surrogate father, whizzing around on a skateboard in between holding the fort backstage and in the bar all summer long. Once, they were travelling home together on the tube, gossiping riotously about the actors in the company, when a lady opposite asked, not all that accommodatingly, if they were a couple. Talbot froze. Conville is a scholar and a gentleman, but he's quite old-fashioned in outward demanour.
"Oh yes," Talbot heard Conville declare, breezily, "we've been together for many years now, haven't we, dear?"
The point about the park is that it's always been a lot better than the critics, including myself, gave it credit for. There was once a very striking black and white Twelfth Night that I was rather snooty about, merely because it replicated the theatre's opening design in 1932. It would still look very good today.
On that long-ago opening night, the actor manager Robert Atkins stood proudly on the greensward with his acting company ranged behind him and boomed, "Every sod on this stage comes from Richmond." He was referring to the turf, of course. Bravo, David.