I am not surprised to see him there, beside me. Where else would he be at a moment like this?

He smiles, briefly, grimly. 'Have I missed anything?'

I shake my head. 'Just the overture – there was a dancing house-elf chorus, consider yourself lucky.'

He slips into the velvet seat and peers down at the stage below us. As the curtains peel back to reveal a comfortable but humble home, with three handsome children arrayed before it, his hands reach forward to rest on the box's railing.

The opening number, 'Three Little Babes' is light and cheery, though it intimates the horror that we know will follow with lyrics such as: 'Three little babes who all unwary/ Live near a Muggle sanctuary/ Hounded by youths who are rough and lairy …'

Potter turns to me and whispers, 'There's no way this can turn out well, is there?'

I shake my head.

The attack on Ariana is at least handled tastefully, reconstructed as a modern ballet to the haunting strains of a boy soprano singing 'Ariana, I just met a girl named Ariana … And suddenly that name, denotes all that is strange, to me …'

Percival's vengeance is swiftly dealt with, to the strains of 'You've Got to Break A Muggle Or Two'. Disturbingly, I find myself humming along with the chorus: 'Auror justice is a lark, You've got to break a Muggle or two ...' until Potter pokes me in the ribs.

The next number is Kendra's lament: 'Summertime', a touching tune with a deceptively simple melody. The lyrics creep under my skin, helped by the voice of the witch playing the role: 'Oh your Daddy is dead, and your Mamma's done running, so hush little baby, don't you cry …' And the final verse: 'One of these mornings, you're gonna wake up healthy, then you'll clasp your wand, and you'll sing like a bird. But till that morning, you keep quiet and calm now, just hush there my darling, don't say a word.'

Potter looks at me, his face gentle. 'That was kind,' he whispers. I nod my surprised agreement.

The moment passes quickly. Kendra's death happens off stage and then the actor playing the young Albus takes to the stage. He's handsome, if a little gormless, and his red hair reminds me strangely of a setter that father used to own. He sings the first part of Albus's keynote song, Fame: 'They'll all look at me/ In books of history/ Forget Flamel and Gryffindor/ Every page will be filled with Dumbledore …'

When Potter starts humming the chorus, I poke him back. Then the two of us pause in our hostilities, mutually frozen by the combined horror of 'I Don't Know How to Love Him' as Albus deals with meeting Gellert, then Gellert's 'If I Were a Witch Man' in reply.

'Do I want to know what a biddy biddy bum is?' Potter asks me, his voice closer to fear than I have ever heard it. I assure him he doesn't.

And now the moment I have been dreading since Dennis Creevey first warned me about the score: the Gellert and Albus medley. It begins harmlessly enough with Gellert's Willkommen, segues into Albus's assurances that life should be a Cabaret, in which he plans to send his brother and sister away so that he and his young lover can move forward with their plans, before they shift into their duet.

It is just as I had feared, the audience in the cheap seats beneath us are enthralled. And who can blame them?

'The flames are at bay now, they've loosened their ties, But how can we truly be free? Till we heed Destiny, Arise, arise! Tomorrow belongs, tomorrow belongs! Tomorrow belongs to me!'

I pity the Muggle that walks in now. But Creevey has not told me what comes next, and my fears are all allayed when Albus and Gellert turn on each other, while Ariana bounces between them to the strains of 'Pinball Wizards'. In the aftermath of her death, while Albus clutches her and Aberforth sings the lament 'Memory' – 'Cursed by pride and/ Filled with rage – they don't see what they're breaking/ A curse goes flying/ And a girl lies dying/ This is the world they're making'– there is not a dry eye in the hall.

The next years of Albus's life are whipped past in a montage. He cuts off his long locks, singing that he is 'Going to wash that man right out of my hair', then shifts to Hogwarts, where he's welcomed by the staff singing 'Oh What a Beautiful Learning'. I find this hard to take seriously. Aberforth's solo, 'High on a Hill with a Lonely Goatherd', sees Potter collapse into a shaking heap of stifled giggles behind the box's front.

The war against Grindelwald sees a return to form, though, culminating in the delegation of witches and wizards storming the gates of Hogwarts: 'Do you hear the people sing?/ Singing the song of desperate times/ It is the music of a people who are dying for your crimes/ When will you take up the fight/ And release us all from fear?/ Surely you could haul him down and bring victory near!'

I am embarrassed to admit that I feel the tiniest clutch at my heartstrings during Grindelwald's rendition of 'Don't Cry for Me, For My Misdemeanours', though I shake it off immediately afterwards when the scene switches to Hogwarts and Dumbledore skulks around after Tom Riddle, singing 'Getting to Know You'.

I glance at Potter. Like me, he is starting to fade a little in this mid-act section, before the rousing fanfare of 'Dumbledore, Superstar!' shocks us into wakefulness. The first Voldemort war seems rushed through, until Albus, now played by an older actor, meets with Snape on a blighted moor and the two sing a duet.

I could have died happily without hearing a man dressed as Severus sing 'Looking back, I could have played it differently/ Left my arm quite blank, and so my soul/ But it took time to understand the man/ Now, I fear, I know him so well'. But life has never been charitable to me.

Of course, this is nothing compared to the shock that strikes me when the action reaches 1991 and Dumbledore greets the incoming year with 'Thank Heaven for Little Boys'. Potter turns to me with his jaw practically on his chest, pointing at the stage. A small dark-haired boy in glasses and a matching one with white-blond hair sit on either side of the ersatz Professor.

'If we close our eyes,' I whisper to him, 'it will go away.'

I'm right, our school years are whizzed through, with the only highlights being 'The Bane in Brain' sung by Professor Quirrell and a chorus of centaurs singing 'What a Swell Parting' as they carry Dolores Umbridge off into the distance.

Potter closes his eyes and sticks his fingers in his ears for Elphias Doge's rendition of 'Secret Love', and Albus's response of 'I've Grown Accustomed to Your Face', but we take heart from the knowledge there's only a short time left before the end.

And then we're there – and it's more appalling than I can describe. In a voice broken and filled with despair, the wizard playing Albus pats the me-analogue on the shoulder, then looks out over the audience and sings: 'Isn't it rich?/ Aren't we a pair?/ You here, unable to kill,/ Me in despair./ Send in the clowns …'

I am almost relieved when the old fool topples from the tower and a facsimile of Potter holds the sword of Gryffindor above his head, singing 'As Long As He Needs Me'. Beside me, the original declares, 'That did not happen.'

I nod. I remember.

All around us there is rapturous applause. The two of us look only at each other.

We wait until most people have left before we speak.

'That was one of the most painful experiences of my life,' I confess.

'Top three,' he agrees.

'Nothing but copious amounts of firewhiskey and a vigorous shagging are going to drive that from my mind,' I mutter.

'But I don't like firewhiskey.'

I look up, and the little devil is grinning at me. I grin back, and for the first time in my life, I feel genuinely and simply grateful to Albus Dumbledore.