The trees in Regent’s Park Open Air Theatre have provided a beautiful bucolic backdrop to many a show. But they can be Blair Witch as much as beautiful and this time they lend Benjamin Britten’s adaptation of Henry James’s chiller a shadowy backdrop.

This Regent’s Park Theatre and English National Opera co-production directed by Timothy Sheader and conducted by Toby Purser, squeezes no less than 13 members of the ENO’s orchestra into the East of England country pile in which the ghost story is set.

Rather than Victorian gothic Soutra Gilmour’s design goes for the decaying greenhouse look. Through that glass, which has been made opaque with a frost of dirt and mildew, the musicians are just about discernible. And as the real sun sets behind the copse, pale artificial moonlight slants across the windows through which orphans Miles and Flora (Daniel Alexander Sidhom and Elen Willmer, who appear in alternate performances with Sholto McMillan and Ellie Bradbury) in impeccable white, goody-two-shoes garb.

Meanwhile their new, sweet-natured governess (Anita Watson on the press night) dressed in black bonnet and gown, is brimful of Mary Poppins optimism when we first encounter her.

If Myfanwy Piper’s libretto charts her psychological descent into something much darker, it’s Britten’s score that articulates it with dissonant chords overlaying sweet nursery rhymes in the way that later became the stock-in-trade for composers working on Hollywood horror. Britten bests most of them. The cacophony of jangling bells heard during during the governess’s dream is like a window into the woman’s internal pandemonium. Or are they alarm bells warning of malevolent ghosts and their plot to steal her charges?

Visually the evening never stops being beautiful. Yet the tension generated by the first half dissipates in the second. It is difficult not to compare this ENO co-production with David McVicar’s which played the Coliseum about a decade ago and still haunts the memory of many of those who saw it.

That production never stopped ratcheting up the tension. Here the ghosts of former servant Peter Quint and Miss Jessel (on this night Elgan Llyr Thomas Elin Pritchard) are never quite the terrifying spectres you brace yourself for.

Granted, Sheader boldly delivers insights into the suppressed psycho-sexual undercurrents of the governess. But the line between what exists inside her mind and outside it, is more of a blur than clearly defined ambiguity.

Still, Britten’s score is majestic. And accompanied by the evening chorus of Regent’s Park birds, which adds immeasurably to the rural setting, there can be few better places to hear it.