Singing in the rain: Bye-bye thunder-sheets, fake wind and wobbly cardboard trees - open air opera revels in the real thing. Iain Gale packed his sandwiches

A night at the opera. On the lawn, four men in dinner jackets are coming to the end of their candlelit dinner. Corks pop and glasses clink. But this is not quite Glyndebourne. The tuxedoed threesome are dining eccentrically inside a chandelier-hung horse-box. Only 20 yards away, anticipating the overture just as keenly, sits a young man wearing jeans and a T-shirt, a six-pack of lager at his side. This is opera for the people, and their number is growing all the time.

In the five years since it began, English Heritage's annual season of 'Opera in the Open Air' has mushroomed from two tentative performances of Don Giovanni at Bolsover Castle - acoustic events with an attendance of 250 - to a major musical happening, with full PA, drawing up to 1,200 people an evening. 'We're getting a reputation for making opera more accessible,' says Brendan Wheatley, director of the performing company Opera Box, who approached English Heritage with the idea in 1989. 'Some of the people who come are real opera buffs, and quite a few are there for a good night out. There's none of the discrimination you often get in the opera house with different ticket prices for the stalls, boxes and circle.'

But if leaving the auditorium has advantages, it also has drawbacks. 'In the open, flames and candles have a habit of blowing out,' John Russell, English Heritage's head of concert planning, says. 'But we've never been rained off. The cast are all equipped with cloaks. They just keep singing.'

The series of concerts - this year Die Fledermaus and La Boheme - starts after the major opera houses and Glyndebourne have packed up for the summer. 'Brendan manages to get the odd name from the major companies and some from the choruses,' Russell says. A prime attribute for potential al fresco stars is stamina. 'We've never cancelled,' Wheatley says with pride. 'We've performed in horizontal rain. Even in a high wind, the orchestra (the distinguished Wren Chamber Orchestra) is equipped with special boxes made from fruit crates and clothes pegs for their music. At Carlisle Castle recently there was a storm and the orchestra tent ended up in the moat.'

Castles, ruins and ancient monuments provide the majority of venues for Wheatley's team who are experienced in exploiting their full potential: 'We always have a site meeting with the technicians beforehand to examine the individual property's possibilities. The death scene in Tosca for instance is wonderful. There are always so many walls for her to jump from.' If sufficiently intrepid, the singer herself will jump, but on occasion Wheatley has pushed a dummy off a 60 ft battlement. 'It lands on the stones very realistically, with a dreadful crunch. There's always an audible gasp.'

Audience reaction is vital to Wheatley. 'Performing as we do in authentic period costume, in a historical venue, I think that we really draw people in.' Sometimes though such involvement can be a little too immediate, such as the night when the torch-whirling cadavers of the Wolf's Glen scene in Weber's Der Freischutz disturbed the resident bats, driving them from the battlements directly into the audience.

Wheatley speaks excitedly of the quantity of pyrotechnics he can use outdoors, of singers on real horses and of the performance of Maria Stuarda when a falconer released his bird high above the walls of Framlingham castle. He is continually exploring new ideas, matching venue to opera. 'I'd love to do Donizetti's Elisabetta al Castello di Kenilworth actually at Kenilworth castle,' he enthuses. 'Fidelio and Wagner would be perfect in some of the ruins.' His greatest ambition is to stage a performance of Bellini's Norma, a touching tale of love and death among the druids of Roman-occupied Gaul. The venue? 'Stonehenge.'

'Die Fledermaus' and 'La Boheme' at Battle Abbey, Sussex, 16-17 July. Tickets pounds 20 or pounds 32 for both. Ticketmaster: 071-413 1443. Info: 071-973 3427

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