Macbeth feels like a curious choice for summery, outdoor theatre; surely no other Shakespeare play is quite as concerned with darkness and night. We sit in broad daylight while character bemoans that "dark night strangles" it. Iris Theatre performs in the lovely grounds of St Paul's, the actor's church, a great choice for a comedy or romance. But the rose garden is far more pretty than pestilent, and creating the nefarious, brooding atmosphere the play asks for is hard with hollyhocks bobbing above your head.

Still, suspend your disbelief, eh. But central London does not want to play ball; the night I see it, the poor actors have to compete for the first 20 minutes with what seems to be a nearby open-air disco playing "Freed From Desire" and the Austin Powers theme tune. Ominous it ain't.

The outdoor setting produces more serious problems than that, however: many of the actors seem to be putting all their energy into projection rather than meaning. Their bellowed lines might make it through the wind, but they too often signify nothing; the verse drifts past rather than truly landing, and emotions seem to evaporate into the air like the weird sisters. Add to that the constant moving of the audience round the grounds, letting the pace lag - one of Shakespeare's speediest shows, this comes in at three hours long - and it's hard to remain engaged.

With a cast of only seven, Iris Theatre give it their all with plenty of doubling. Helping this are some elaborate costumes for the witches: they become giant, green-black beetle-like creatures, with bulbous red eyes, elephant-trunk noses or a venus fly-trap head, one walking on stilts. They're fully alien, but rather fun. The second round of prophecies are eerie too, faces pressing through stretched latex panels in a grotesque veined tent, as if spirits are reaching through the fabric of this world; sadly, the creepy voice one actor adopts tips the audience into accidental giggles. It isn't the only time this happens.

Daniel Winder makes some odd choices in his direction. Slo-mo battle scenes are massively naff, not helped by the rather cheap-looking, also beetle-like battle armour they wear. At the banquet where Banquo's enjoyably wild and bloody ghost appears, the lords at the table are two out of the three witches. Why doesn't Lady Macbeth see this? Is it all in Macbeth's head? It's unclear. I also wasn't convinced we needed the necrophilic pawing of poor Macduff's wife as a way to make the audience move to the next scene.

David Hywel Baynes goes from a rather cheery Macbeth to a sickly, ravaged man - after the deed, he rocks with the shock of it, eyelids fluttering, and gets increasingly wild and feverish. Mogali Masuku's Lady Macbeth is an emotional, teary one – we do sense the impact the loss of a child has had on them both – who later becomes fearful of Macbeth's mania. But all too often the acting just isn't really up to the play. There's a lot gesturing and earnestness but Shakespeare feels stubbornly remote, rather than able to speak to us.

Macbeth runs at St Paul's Church until 29 July.