Observations: All hail Sir John Tomlinson, opera's king of bass

Help! – is there a bass-baritone in the house? When one of these rare beasts falls sick, as has just happened at Covent Garden, the search for a replacement becomes a nail-biter, particularly when the role is as demanding as that of King Marke in Tristan und Isolde. It just so happened that the perfect replacement was indeed in the house, just singing another role on other nights. Step forward Sir John Tomlinson, the Wotan of many critics' dreams – "magnificent", "towering", and "majestic" being the commonest epithets – and therefore the dream King Marke too. His magnificent etc performance as the Grand Inquisitor in Covent Garden's current Don Carlos will, from 29 September onwards, be complemented by this tormented royal victim.

Two years ago this unassuming Accrington boomer saved the Covent Garden Ring by stepping in as Wotan when Bryn Terfel cried off, becoming the star of the show at a time when everyone had assumed he was starting his downhill glide. Will he go on for ever? A low, rumbling laugh: "I'm now 63, and supposed to be slowing down. It's not the voice – that's just fine – it's the energy levels, and these roles take an immense amount of that. There are no Wotans in my schedule, but I have not said I will not do any more – I think there are a few more in me." Who will be the next Wotan after him and Terfel? He laughs again. "There are two or three people I know, but I don't want to put pressure on them by saying they are the next. It's not fair on them." What about Matthew Rose, who seems to have exactly the right physical and vocal heft? "No, I won't be drawn." But I sense we're on the right track.

'Tristan und Isolde' is at the ROH, London WC2 (020-7304 4000), from 29 September