Alan Ayckbourn's acerbic intertwined 1990-set comedies House and Garden find their ideal home in the Watermill's country house setting and glorious garden. Master of structure, Ayckbourn clearly relishes challenging actors to perform both plays simultaneously – their exit in one is their entrance in the other – and challenging audiences to keep up. Director Elizabeth Freestone choreographs her surefooted cast through his serpentine conga to maximum comic effect. It's bittersweet or often just bitter.

Designer Neil Irish opens out the Watermill's deceptively small stage to suggest a rambling country house with conservatory doors leading to the hedged garden; and places centre-garden the fountain that, along with the Watermill's real hedges, and the village fete stalls and maypole, is almost an extra character.

House focuses on the amorous and political wheeling and dealing of the hereditary owners of both house and garden; only in Garden do you get to make the proper acquaintance of the staff who serve at their luncheon party as tirelessly as they eavesdrop on them. And the muddied waters of relationships in House become literally mucky in Garden, thanks to weather as British as the annual village fete, first set up, then washed out, in the grounds.

Stately home owner, wannabe member of parliament and serial adulterer Teddy is the rotter whose fling with next door's Joanna causes pain, misery and hilarity, indoors and out. Tim Treloar is simultaneously loathsome and loveable, expertly inept, both verbally and physically. This is a comedy of routines, both slapstick and verbal and Teddy's inability to speak to luscious French starlet Lucille (enchanting Nanou Harry), guest of honour at the fete, in her own language is as hilarious as their sexual encounter in a collapsing rain-sodden tent.

Teresa Banham finds the genuine pain in his wronged wife Trish, from a long line of long-suffering navy wives, the worm turning to surreal comic effect. The isolation, the sterile marriages of these stiff upper-lipped members of the county set is palpable. Trish may hold it together but Joanna is falling apart. Cate Hamer finds the comedy in her sexually voracious mistress and the tragedy in her descent into dishevelled delirium. Ayckbourn ensures maximum audience sympathy for wronged husband Giles, understanding, loving, humane – he's even the much-loved village doctor! Robert Mountford plays him with appealing earnestness and understanding.

His son Jake, mooning after next door's daughter Sally, is a chip off the old block. Imram Momen's awkward puppy love for Grace Cheatle's equally convincing bright, coltish sixth former is a sweet delight. By contrast, Darrell Brockis's shiny, oily political fixer Gavin, Teddy's old school-friend, is a sinister, lizard-like predator playing with political wannabe Sally.

There are awkward touches of class snobbery. Trish is offhand with her staff and mistaking Lucille's agent Fran for a driver, does not include her at the lunch table. Jessica Woo's frazzled Fran trying to keep gleeful alcoholic Lucille from the bottle – especially the whisky on the hoopla – comes into her own in Garden.

Seeing both plays (which I strongly recommend) provides the fun and satisfaction of getting closer to characters you met in one, and meeting proper characters who barely figure in the other. There's also the dawning realisation that an intriguing loose end in one is being expertly tied up in the other.

In Garden you get to eavesdrop on the resolution of the complicated domestic arrangements of House's staff: Gary Pillai's hilariously monosyllabic Warn, the hunky gardener channelling Mellors, Melody Brown's frazzled housekeeper Izzie and her mini-skirted daughter Pearl (Louise Coulthard, oozing earthiness as a maid in the house and fortune-teller at the fete). And the dysfunctional husband-and-wife team of Gareth Kennerley's infuriatingly bossy Barry and Sally Tatum's long-suffering Lindy, the anoraks organising the fete, are literally the mainstay of Garden, though they make only a fleeting appearance in House.

And only in Garden do you get to coo over the troupe of local children who expertly dance around the maypole and beam with delight as Gavin judges their fancy dress competition. A crowning delight for a perfect summer's evening's entertainment.

House and Garden run at the Watermill Theatre until 1 July.