Au Revoir Parapluie, Sadler's Wells, London <br/>Roadkill, Barbican Pit, London

With Charlie Chaplin as a grandfather and both his parents circus innovators, there was little choice for James Thiérrée but to run away and join a firm of accountants. Either that or pick up the baton and develop his own take on the family aesthetic.

Au Revoir Parapluie is his third stage show since departing the cast of his parents' travelling circus, the tiny but mould-breaking Cirque Imaginaire, begun in the 1970s by Victoria Chaplin (acrobatic last daughter of Charlie) and her husband, the gentle clowning genius Jean-Baptiste Thiérrée. The shows were small-scale, exquisite, full of innocent glee and poetic absurdity. A choir of live ducks provided the finale to one memorable production. Fantastic sculptures emerged from piles of old chairs.

Au Revoir Parapluie, like James Thiérrée's earlier go-it-alone projects, taps the same vein of visual playfulness, but is conceived to be played in bigger theatres. The show's presiding image is a vast skein of rope suspended from a giant rusty crane-hook. You think rain, Rapunzel hair, strings of human veins, then a ladleful of cooked spaghetti, as the multiple strands descend in a plumply tangled heap. The more monkey-toed members of the cast of five slither up and down its shafts, swing them, fight with them, disappear under them, but it's never their skill so much as the sheer fact and scale of this extraordinary octopus thing that dominates the imagination. It's the same when Thiérrée appears riding a large metal wheeled structure, a cross between a cherry picker and a see-saw. I stopped seeing people and only saw things.

Apparatus has always had a place in circus, and alternative circus too, but it strikes me that the increasing scale and technical input of Thiérrée's shows (encouraged by the big budgets he now has to play with) risk squashing the magic from his work.

This show also sags – too often in what feels a long 80 minutes – every time Thiérrée himself is not on stage. Clearly he likes the company of a group, but, alas, the talents of his co-performers are a poor match for his own. The straight man's dud magic and painfully heavy backflips are meant to be comically bad, but the women – presented as a bunch of scuzzy flatmates – are just plain irritating. They include a Puckish, androgynous one given to grinning insanely while spinning like a frisbee, and another who will not stop singing. There's relief when she is bundled into the wings and gagged.

Everything changes when Thiérrée steps forward for a solo turn, and the messy business around him melts away. A primary joy is the sheer beauty of the man: you don't expect a clown to be so lovely. Elegant as a mannequin as he obsessively folds up his jacket and continues to fold up himself, or scolds wilful feet that take strides either too large or too tiny (echoes of grand-père in his dancer's delicacy, half self-mocking, half serious), Thiérrée is magical to watch, and literally enchanting.

There is finely nuanced comedy too as he lip-synchs to impassioned gypsy fiddling, or rolls a Bach melody into a ball and eats it (we hear its distant rumbling in his stomach). It's this knack for connecting the unconnectable, a kind of kooky synaesthesia, that both builds on the family tradition and distinguishes him. If only the walls would close in, the audience shrink, the redundant cast sign off and the pantechnicon arrive empty, we could see Thiérrée's extraordinary talent in its best light.

Stage props perform a distinctly sinister function in Roadkill, an intriguing physical-theatre thriller from a Brisbane-based trio called Splintergroup, brought to the UK by Dance Umbrella. Tapping into the myths and paranoia inspired by the vast red-dirt emptiness of the Australian outback, Roadkill invites us into the predicament of a couple whose car breaks down in the middle of nowhere. Half-realistic, half hallucinatory, the story is told through the experiences of each of three protagonists: the driver, his girlfriend, and the aggressively friendly stranger who insists on helping them out. Who is most suspicious of whom, you are led to wonder.

It's a socking idea, executed with flair: the set is just the car, and a rundown phone kiosk. The car radio provides much of the soundtrack, and lighting marks the progress of night into day into night again, as well as providing look-behind-you tension. One duet – violent or consensual is hard to tell – is lit solely with a hand-held torch, yielding only intermittent clues as to who's still standing.

The passing of slow time presents a tactical problem, though. How much birdsong, staring into space and fiddling with a flat mobile can a drama take? The show also lacks dramatic shape – too many episodes, too many climaxes. That said, however, some of those climaxes are riveting: the cringemaking moment when the lovers realise their energetic love-making in the back of the car is being watched; the frenzied sequence when the stranger appears to have stolen the car (the impression of speed achieved by performers dashing past the stationary car with eucalyptus branches; the crash that makes the bodies inside lurch horribly around in slow motion like a TV seatbelt ad.

Are these the ravings of heat-stroke, or alternative endings to a human tragedy? It's left to you to decide, as the boyfriend arranges pebbles over his lover's body, produces a toy car (a miniature version of his own, with working headlamps) and proceeds to drive it over the rocky passes of her thighs and stomach. This may be the ultimate wishful hallucination: a car that works. Recommended.

'Au Revoir Parapluie' (0844 412 4300) to 10 Nov; 'Roadkill' (0845 120 7550) also to 10 Nov