'We will continue to pay to see Shane MacGowan, provided he does his job'

It was the whirling lager bottle that did it. I'd gone to see The Pogues perform at Brixton Academy for the fourth year running, and this year I'd taken my two eldest children. They'd been listening to the music of the London-Irish band since they were in romper suits, and they'd heard about the legendary Christmas concerts, where everyone sings along to the bittersweet lyrics of "Fairytale of New York" and weeps when the snow cascades down.

The Academy is a staggeringly down-at-heel music venue, where the smell of vomit hits you as you walk in, the queues at the bar resemble the crush at the start of the London Marathon, and the sucky floor seems to drag at your shoes like the great bog of Doone; but the sight-lines are terrific, and the band, especially the masterly James Fearnley on accordion, played as well as, or better than, I've ever heard them.

And then there was Shane MacGowan, for 20 years the walking, staggering epitome of the talented piss artist, the drunken angel, the self-destructive boho genius who needs a bottle of Jameson inside him before he can express himself, who creates songs of immense beauty from the shambolic, toothless wreckage of his own life... Well, Shane was extremely drunk.

I realise that this revelation isn't going to win prizes for shock value, but I've seen The Pogues many times, and something changed inside me as I watched their leader screw up a good concert. In the past, an understanding has existed between the band and their fans. We will continue to pay to see Shane as a wayward songwriting genius, amusingly, triumphantly sloshed day and night, provided he does his job and sings the words without expiring or throwing up over the front row. MacGowan has, in the past, displayed a mysterious talent for remembering all the words to his songs, even when he is legless; then he'll forget the opening line of "Fairytale", and everyone will say, "Isn't he a caution?" and go home delighted and tell their friends they've seen the wild Pogue in action.

It's not like that any more. He wandered on to the Academy stage in a long, black tramp's overcoat, holding a bottle of wine, a table of drinks in plastic cups was waiting beside his mic, and he kicked off with "Streams of Whiskey". It's been a while since MacGowan was able to hit any note, but he carried the first few tunes by brute force, and the band tootled and banjoed and accordioned with an attack that carried the singer's shouty-slurry vocals.

After the half-way mark, though, things went to hell. MacGowan's timing packed up. He missed the start note of choruses. He forgot words, then whole lines. In one song, he became confused and mumbled the lyrics of the previous song until told to desist by his tin whistle player. It was ghastly. The crowd – a well-oiled, slam-dancing throng, but warm-hearted with it – joyously sang along to "Sally MacLennane", but Shane screwed up the chorus by starting too early.

As the band played at their usual breakneck speed, and the singer struggled (and failed) to keep up, I realised I was sick of feeling fond and indulgent towards artists who cause such destruction to their own works. MacGowan's songs were always so lyrically crammed, they were hard for anyone to sing but himself. Now it was pathetic to watch him, during the super-fast "Bottle of Smoke", trying to recall the odd word of verse or chorus, as the tune galloped past him. Foggily preparing to sing "The Irish Rover", he mumbled, "This is for Ronnie Drew," the late lead singer of The Dubliners, who once duetted with The Pogues on this very song. By the end of it, I wondered what Ronnie D would have thought of his younger friend's rubbish performance, casually parodying the worst drunken-Mick stereotype that the Irish mostly threw off in the last century.

People began to throw plastic cups. McGowan, trying to sing a cappella, swatted them away like Frankenstein's monster swatting away the torches of villagers. Then the guy in front of me flung a lager bottle that described a perfect arc, and landed, like a perfect darts throw, neck-side down in MacGowan's coat pocket. The crowd roared with delight. MacGowan looked up, briefly animated. By God, he'd been right. He knew he could win them over, if only the music stopped racing along. He knew he still had the old magic...