At first glance, the award-winning designer’s garden is deceptively traditional and genteel, boasting a beautiful octagonal folly and sunken Italianate pond.
But every 15 minutes, like clockwork, the maverick Irishman’s tranquil garden will be transformed into a madcap, mechanical, flight of fancy with whole flowerbeds rising from the ground.
Box balls will bob up and down; bay trees will twirl; foliage will be trimmed by mechanical shears; and a set of patio furniture will emerge through a trapdoor from its subterranean storage.
The folly will undergo a complete change of appearance during the outlandish, tongue-in-cheek two and a half minute routine.
And a wooden shed will house an inventor’s crazy contraptions made of old bicycle wheels, cogs, and bits of string that also come alive.
Gavin, 51, said that his latest typically extravagant effort was inspired by the Guinness clock from the 1951 Festival of Britain and the odd contraptions drawn by legendary cartoonist William Heath Robinson.
Every quarter of the hour, the mechanism of the 7.5m-high Guinness ‘Crazy Clock’ in Battersea Park made it play fairground music, while a zookeeper and cast of animals performed a routine.
Heath Robinson, who died in 1944, was known as ‘The Gadget King’ for his eccentric illustrations of absurdly complicated machines made to achieve the simplest of tasks.
TV presenter Gavin, who won Chelsea gold with his spectacular 2011 floating Irish Sky Garden, said: “Harrods sent me away with a strong brief to come up with something British.
“I did not just want to do a traditional, beautiful garden. I wanted to include a sense of playfulness but it was making me demented trying to figure it out.
“I was at Hampton Court with a friend who said that something reminded them of Heath Robinson and the idea for the entire garden came to me like a thunderbolt.
“On the surface, it is a genteel garden, with the folly representing elegant Victorian or Edwardian life.
“But every 15 minutes it will spring into life as a Chitty Chitty Bang Bang-style flight of fancy where all the plants move around.
“I don’t want to spoil the surprise but the folly physically changes as plants around it rise up to window level and furniture comes out from underground.
“The mechanics of the garden are not easy to do at all because it has to work every single time.
“I was slightly worried about telling Harrods but they adored the idea.”
The ‘Harrods Eccentric British Garden’ is believed to be costing upwards of £250,000 (€350,000) to create.
Gavin is also designing matching in-store displays and terrace gardens to coincide with next May’s Royal Horticultural Society flagship event.
It is Gavin’s first Chelsea garden since his 2012 seven-storey, pyramid-shaped ‘Magical Tower Garden’.
The showman had 75 Chelsea pensioners aged between 69 and 93 line up on his tower, which was awarded a special honour, the Most Creative Show Garden prize.
His 2011 design that struck gold was was inspired by hit movie Avatar and came complete with a bright pink pod hoisted 25m up in the air by a crane.
Gavin, who did not even visit Chelsea last year, said: “It will be nice to be back.
“I hope parents will bring their kids over to my garden to be entertained every 15 minutes.
“I loved the idea of the Guinness Clock where people queued up waiting to watch the routine. And if I can capture just a little bit of the spirit of Heath Robinson, who was a lovely man as well as such a talented illustrator, I will be delighted.”
Chelsea Flower Show takes place between May 24 and 28, with tickets on sale next month.