Richard o Brien Rocky Horror Picture Show

So it is that the 73-year-old impresario is facing me in an unexceptional green room at the Playhouse Theatre. His disposition is kindly and he’s dressed in black; his head instantly recognisable and entirely hairless. He’s previously described himself as 70 per cent man, 30 per cent woman. “I’m kind of third sex in a way, ” he says now. “I’m in the middle there somewhere. There’s a lot of male in me, there’s no doubt about that, but there’s a lot of girl in me as well.”

His third wife, Sabrina Graf, whom he married in 2013 and who, at 41, is more than 30 years his junior, has also put a number on it. “My lovely Sabrina said, ‘I think it’s probably either 50-50 or 60-40. 60 male, 40 female. I said 70-30 because there is a lot of male in me but I don’t know how much of that has been imposed upon me - is it nature or nurture? We’ll never know.”

O’Brien hasn’t ever wanted to reverse his gender at the hands of a surgeon though. “I don’t see how that could ever work because I would never be a woman, I could only be an idea of a woman, ” he says. But he does take a daily oestrogen pill. “I didn’t like being male and I knew I could never be a female, ” he explains. “I thought if I could just be in the centre somewhere I wouldn’t have to deal with the conflict anymore. Because it was a bit like being schizophrenic.”

It would, O’Brien says, have made more sense had he been born a woman. “If I’d woken up in the morning and suddenly discovered through some magical experience in the night that I was a woman, I’d have gone, ‘That’s the way it should have been.’” But he finally became comfortable with himself about 10 years ago. “I said, ‘I’m transgender. OK. Accept it for yourself and the rest of you accept it too. Get on with it.’”

Now, he says, he’s happier than he’s ever been. “The lovely thing about Sabrina is she doesn’t allow me to be the way I am, she celebrates the way I am. Which is delightful. She adores me and I adore her.”

As for his sexuality, O’Brien doesn’t wish to be pigeon-holed. He’s slept with men, women and transsexuals (although one thing he won’t do is cheat). “Sex is the most wonderful reaffirmation of the life force and recreational sex is divine. As long as everybody’s having a nice time, I don’t see any problems, ” he says.

O’Brien has three children: a son from his first marriage to Kimmy Wong, whom he met when they appeared together in the musical Hair in the early 70s, and a son and daughter from his second marriage, to Jane Moss, a designer. He met Graf, who is from Germany, when she was waiting outside the stage door after coming to watch him in Chitty Chitty Bang Bang. He was 60 and playing the Child Catcher. They later became lovers and would meet for weekends in European cities.

“I really was pushing her away, ” he insists. “I said, ‘I’m far too old for you, you’re too young and too lovely and too gorgeous and I don’t want you to watch me growing old and becoming incontinent and brain dead.’” But eventually he asked her to marry him in a New Zealand dressing room in 2012.