The ENO show's not over until the understudy's understudy can't sing

It was, appropriately enough for this most turbulently aquatic opera, the perfect storm: a leading man too ill to perform, his understudy and leading lady laid low by the same bug. One of the highlights of English National Opera's summer season, Penny Woolcock's lavish new production of The Pearl Fishers, features stunning projections, billowing waves of silk and teeming crowd scenes. Window-dressing aside, the main attraction of Bizet's relatively rarely performed score is the famous duet between the two male leads, Nadir and Zurga. And the main attraction at the London Coliseum, to the extent that he is the only cast member named in the brochure, is that Alfie Boe, the "Tony Award- winning tenor", sings Nadir.

Except that, on Saturday night, he didn't. As the lights dipped, John McMurray, head of casting at ENO, came on stage to inform the audience that Boe was suffering from a chest infection and would not be singing. As one who had bought four tickets on the strength of Boe's name on the billboard, I joined in the groans. There was more to come. "Sometimes an illness takes a grip on an opera company", said Mr McMurray. "And this is one of those times." Boe's understudy, Christopher Steele, was also suffering from a throat infection, as was lead soprano Hanan Alattar, singing the romantic heroine, Leila.

These last two, we were told, would battle through as best they could. Very good of them, but you might argue that, at £90 a ticket in the stalls, battling through wasn't quite what you'd paid for. Still, we were a captive audience by that point, safely ensconced in our seats, away from the box-office staff. Only a few voted with their feet, heading straight back out of the Coliseum, just in time for Steven Gerrard's goal.

As the first act wore on, it was plain that Steele, clutching a bottle of water, was struggling. He manfully roared out his duet with Quinn Kelsey's Zurga but the "Je crois entendre encore" aria was painful. It was no surprise when, after the interval, McMurray appeared again to announce that Steele was retiring for the evening, lest he damage his voice permanently.

Time to drop the curtain? Not quite, McMurray had another trick up his sleeve – not the official understudy's understudy, sadly, but a member of the chorus, David Newman. He'd sung the role before, we were told vaguely, not in English translation but, you know, he'd have a go. He proceeded to sing from a book at the side of the stage while Steele mimed, thrusting poor Alattar into a whole new romantic threesome. Scenes of high passion took on a bizarre ventriloquist quality as Nadir embraced Leila while apparently throwing his voice from somewhere over by the ice-cream seller. In the stalls, Nadir began to take on a whole new meaning.

At the end, there was applause, tinged with relief. ENO deserve credit for their efforts, as do Steele, Newman and Alattar for performing in near impossible circumstances. But at what point does the determination of the company to carry on become a feat of endurance for the audience? At what point does "the show must go on" tip over into "must the show go on"?

Illness happens – and in the case of highly trained singers and dancers, quite frequently. So why not have better contingencies in place? There were around 50 members of the company on stage on Saturday and only three significant singing roles. Would an extra understudy for Nadir have been too much? Was there no other lead tenor available in London who could sing the role?

Front of house stated on the night that since the role was "covered" there would be no refunds. But was the role covered satisfactorily, when judged against the standards (and prices) of a world-class opera company? Of course not. Unlike concert promoters, who must produce the real deal or refund the masses, opera, ballet and theatre companies can argue that audiences have booked to see a show, not an individual star. In an age of celebrity casting and big-name stars – David Tennant and Martine McCutcheon to name but two notorious no-shows – it's disingenuous to suggest such a thing.

Refunds are only offered if the performance is cancelled in whole or in part by ENO. This is, according to a spokesperson, in line with the policy of every other opera house across the world. Not quite. When Placido Domingo dropped out of a wildly anticipated Tamerlano at the Royal Opera House earlier this year, Covent Garden offered ticket-holders a 20 per cent credit note. Boe is no Domingo, but he was undoubtedly the star draw of this particular production: ENO audiences deserve a similar goodwill gesture.

The opening run of The Pearl Fishers lasts for less than a month and consists of just 11 performances. Since a refund is out of the question, perhaps the ENO might consider an extra show for dissatisfied customers to see it again in its full glory, free of charge. At the very least, a programme voucher might help to entice me back to the Coliseum – that £5 I spent to find out who was singing what turned out to be a real waste of money.

The understudy's moment of truth: 'I got the message... David Tennant can't make it'

Edward Bennett, 31, shot to fame in the Royal Shakespeare Company's Hamlet, at the Novello Theatre, London, when he was informed, hours before a show in December 2008, that David Tennant would be unable to play his role due to a bad back. Bennett gained huge critical acclaim for his month-long performance in the leading role.

He said: "It was Monday, late morning, when the stage manager phoned me up about that evening's performance. The job of a stage manager is to be very calm, for their voice not to betray anything, so I got a message saying 'Just to let you know that David Tennant can't make it so could you come in by 3pm?'

"I remember being unable to tie my shoelaces. It was like being in shock – it was too big to take in. I phoned my agent, my best friend, and my parents. I wasn't in a rush to get to the theatre; I thought, 'If I'm going to die, I may as well take it easy'. When I got there, I went over the lines as much as I could.

"I knew it was going to be OK but I was very, very nervous. I felt really bad for David; I didn't want to let anyone down; I was scared. At the time, I thought David would be away for one night because the following night was press night.

"I heard (the RSC's associate director) Greg Doran make the announcement just before the show. He went out in front of the audience to tell them what was happening and I heard a huge groan when he told them David Tennant wasn't going on. He then made a beautiful speech, it was so kind. He said 'We have this exceptional actor who is the understudy'. He came back off stage and just for a moment, he held my hand.

"The show started and I can't remember any of it really, until the end. I got a standing ovation and I remember Patrick Stewart beckoning me on stage.

"I went out and got absolutely hammered with a friend that night only to be told the next morning I would be doing it again, this time for the press. It was then that it became very real. I was very ill for the first week: my voice was going and I had horrific flu. I was trying so hard and acting is about the only profession in which the harder you try, the worse you are.

"The experience has perhaps led to TV work. It got me noticed and I've now got that on my CV. I also got some film auditions and got to meet casting directors. I don't want that to be my legacy, although I will forever be grateful I got a chance to play the part, and that David Tennant's back was not irretrievably buggered."