It clocks in at nearly two hours, including ad breaks. And almost everyone ever involved in Potter – again with one exception – appears to have been hauled in for a grilling. It is also, in its initial segments at least, rather cloying, with the saccharine, Vaseline-on-the-lens ambience of a John Lewis Christmas ad. We see Emma Watson (Hermione Granger) become over-wrought as she receives a magical invitation to board a train at Platform 9 and 3/4 at King’s Cross. Joining her there are other Potter veterans such as Coltrane and Matthew Lewis (Neville Longbottom).

They arrive at the Great Hall of Hogwarts, which, as luck would have it, has been perfectly preserved as a visitor attraction at Warner Brothers’ Studio Tour London. Beneath floating candles, dancers sway while Potter gang catch sight of one another and hug. It’s a bit of a Harry Hodgepodge – 50 per cent sparkly Hollywood overkill, 50 per cent old mates meeting up and wondering if it’s too early to get the drinks in.
 
The film settles down into a more conventional “making of” documentary as Radcliffe arrives and he, Watson and Grint retire to the Gryffindor common room to reminisce about forming the ultimate children’s fantasy power-trio in Harry, Hermione and Ron Weasley. Behind-the-scenes footage from the early Potter films reminds us how young they were at the start of their adventures together and how much they have grown, even if they’re still only in their early thirties (Radcliffe’s movie-star quiff looks like it deserves an anniversary special all its own).
 
They have maintained a genuine bond, as becomes clear towards the end when Grint breaks down slightly and tells Watson he loves her (in the strictly platonic sense). These exchanges are padded out with interviews with alumni of the expanded Potterverse. Gary Oldman recalls signing up to 2004’s Harry Potter and the Prisoner of Azkaban because he wanted to work with director Alfonso Cuarón.
 
Ralph Fiennes, for his part, speaks warmly of the late Alan Rickman, saying he was intimidated by Rickman’s “expert precision” and that they went “toe to toe” in their face-offs as Voldemort and Severus Snape. And it is revealed that Rickman persuaded JK Rowling to reveal to him how the Potter saga ended (she was still writing it when the early films came out) and that he layered that forbidden knowledge into his performances.

Yet Return to Hogwarts is also hugely sanitised. There is no scrutiny of the series’s tendency towards stereotyping: the Chinese character named “Cho Chang”, the dim Irish Hogwarts student who blows himself up with explosives, the money-hoarding goblin bankers who, it has been claimed, draw on anti-semitic archetypes.

Nor are there any references to the dreadful Fantastic Beasts spin-off franchise, which axed Johnny Depp after the actor lost his libel case over allegations of assault towards ex-wife Amber Heard and which has removed any reference to JK Rowling in trailers for the forthcoming Fantastic Beasts: The Secrets of Dumbledore. And the reunion glosses over the impact on its young stars of overnight celebrity, even though Watson and Grint reveal that, as they became older and the films grew in popularity, they considered walking away.

“I can see that at times I was lonely,” says Watson, of the making of the fifth instalment, 2007’s Harry Potter and the Order of the Phoenix. “I was scared. ‘This is kind of for ever now’.”

“I had similar feelings,” says Grint. “”What would it be like if I called it a day?”

“As a 14-year-old I was never going to turn around to another 14-year-old and ask, ‘are you okay?’,” chimes in Radcliffe. 
Yet even this dark episode is tied off with a bow. “The fame thing had finally hit home in a real way,” says Watson. However, she adds that she had stayed the course for the fans – “they genuinely wanted you to succeed”. And with a single wave of a wand, any potential downsides of childhood fame are vanquished.

Potterheads here for wizardly lore rather than authentic human interest stories will lap up Return to Hogwarts and its many nuggets about the making of the movies. Chris Columbus, who directed the first two, recalls that the search for an actor to play Harry “was insane”. Cuarón says Prisoner of Azkaban was darker than the previous Potter pictures because it was a coming-of-age film. But what’s ultimately missing is any genuine insight into Harry and what it was about him that transfixed so many readers. And, to be honest, only JK Rowling could have offered that kind of insight.

On Sky Max on New Year’s Day at 8pm