Sir Tom Allen is still fresh, four decades on Where to begin with the achievements of Sir Tom Allen? As Britain's best-loved opera singer, and as the real-life inspiration for Billy Elliot - hailing from a mining family up north, with no expectations of stardom - he embodies the sort of story dreams are made of. He has created a parallel career as a director, and has just been appointed Chancellor of Durham University, but tonight will see a different culmination: when he walks on stage at Covent Garden in London as Don Alfonso in Jonathan Miller's wickedly knowing Cosi fan tutte, it will mark the 40th anniversary of his first appearance there. Ruthlessly manipulative beneath his genially avuncular façade, Sir Tom's Don Alfonso is incarnated with such authority that it's now hard to accept anyone else in the role. And he regards this comedy as profound: "It's like an atom bomb on stage, everything gets blown asunder, and each time I do this show it gets more interesting. Even in rehearsal this morning I had a new thought. In Mozart's day, he thinks, the crossed pairs of lovers would have gritted their teeth and gone back to their original partners, but not today: "Now we think, 'I've just faced Becher's Brook in my marriage, so I'm going to duck out'. We tend to give up quickly, rather than work at things. 'Cosi Fan Tutte', Royal Opera House, London WC2 (020 7240 1200) tonight to 13 February