Someone is heading purposefully for my table, sweating, beaming and bearing a large plate of food. But it’s the size of his girth that diverts me: his dressing gown can’t cope. No sooner has Christopher Purves plumped himself down on the nearest seat than he proudly raises his under-garments, encouraging me to survey all that lies beneath. I make suitably admiring noises and can’t help laughing.

What I have just beheld is a rippling layer of synthetic blubber. Purves calls it a “sort of latex suit” and says he only has half of it on: the top must be somewhere in the dressing room or rehearsal hall, which explains why his posterior looks so much larger than his chest.

Only a Falstaff could be like this – lusty, overweight, a shade ridiculous but palpably at peace with himself. And only someone as attuned as Purves to the “donning” of character could look so at home in a half-costume that might otherwise be an embarrassment. Purves was a surprise choice to sing the title role in Glyndebourne’s new Falstaff, which opens the English country house opera season next Thursday. He is not the big barrel of a man we associate with Verdi’s fat knight. Nor is he exactly a household name in the opera world. But recent seasons have seen a steady flow of outstanding performances – above all in the title role of Berg’s Wozzeck for Welsh National Opera, with the same conductor-director partnership of Vladimir Jurowski and Richard Jones that is tackling Glyndebourne’s Falstaff. Few British singers deserve the description of “stage animal” as readily as Purves. Whatever role he adopts, it’s all in the acting – which explains why his performance promises to be so interesting.

Recognition has taken time to come, at least in opera. Purves is that rare beast – a singer who made his name in pop before crossing over to classical. He first won fame as a member of the rock group Harvey and the Wallbangers. “Well, not strictly rock and roll,” he corrects me, “in that we didn’t have long hair or play big guitar solos. We were slightly poking fun at that – there was an element of pastiche in what we did. We were an all-singing, all-dancing group, with trumpet, sax, trombone, guitar, keyboards and drums. We did everything off our own bat – we weren’t commercial at all.”

But surely, as a member of a 1980s touring and recording outfit, he must have been as much a part of the sex and drugs culture as any other band? “I can’t possibly comment on that – my parents might read this.” Purves’s smile says it all. Now a contented family man, he believes his wilder days gave him one durable asset: the ability not to be scared of an audience, which makes him the envy of many operatic colleagues. “What I found [with Harvey and the Wallbangers] was that, if you seemed to be enjoying what you were doing, that would transmit itself to the audience. If you’re having fun, they will too. So when I moved into the classical field, it wasn’t the audience I was worried about, but the fact that I didn’t have confidence in my ability as a singer. I hadn’t had the training and rigorous apprenticeship others in my field had attained.”

Listening to Purves’s clean-cut baritone, it’s hard to believe he was once a rocker, but it’s not strictly true that he missed out on vocal training. He had one of the best vocal educations that exist – as a chorister at King’s College, Cambridge, later touring the world with a generation of performers that included Simon Russell Beale and Simon Keenlyside.

Purves says that his King’s experience was double-edged. “The English choral tradition is damaging to anyone who wants to sound individual, because the whole idea is to sublimate your personality for the greater glory of the body of sound. It took me a long time to break that cycle. But give me a part-song now and I’d go straight back to the way I sang at King’s.”

Therein lies the clue to one of Purves’s other great assets – the sheer breadth of his repertoire. Look up his discography and you’ll see he is as prized for his Bach, Handel and Purcell as he is for Modernists such as Luigi Nono, whose Al gran sole is next in line on his engagement calendar at this summer’s Salzburg festival.

In that context, Falstaff and Wozzeck lie somewhere in the middle of his frame – though Berg’s downtrodden victim, a role to which Purves returns in September for the first revival of WNO’s production, could hardly be more different from Verdi’s corpulent chevalier.

Purves points out that Falstaff and Wozzeck were written barely 30 years apart, “but seem centuries away from each other. With Wozzeck you’re dealing with a timid animal that could kill. You’re learning how to strip away all extraneous emotion and movement. He is the bare bones of a man, the opposite to Falstaff. Falstaff is a generous bear: you have to learn the largesse and flamboyance. Unlike Ford [the other baritone role in Falstaff], where there’s not much filling for the character, Falstaff has everything – extravagance, seduction, vanity, cruelty, introspection, bitterness, generosity.”

Not such a modern character, then, as Wozzeck? Maybe not, “but nor is he antique,” counters Purves. “As in all the greatest art, there’s something universal and timeless about him. Falstaff is a modern man in an old-fashioned mindset. He can’t grasp that he is not keeping up with a changing world. It’s a generational thing. You look at your children and say ‘In my day we didn’t talk like that...’.”

Lunch is over. The rehearsal bell has sounded. Purves lifts his girth, bids me farewell and waddles off to Glyndebourne’s version of the Garter Inn.

Glyndebourne Festival Opera’s new production of Falstaff runs from May 21 to July 11