The dusty earth flies up in clouds behind John Hinchliff as he strides down the path between rows of blackcurrant bushes, their leaves wilting in the afternoon heat and exhausted branches weighed down with clusters of glossy berries. "We could do with some rain," he says. "I am obsessed by the weather. My entire life is dependent on it." This year's blackcurrant crop is looking bountiful, but it wasn't looking so promising earlier in the year. "Blackcurrants need a cold winter, so we started the year with a full expectation of lovely blackcurrants," says Hinchliff. "Then we went into one of the most miserable springs, which was worrying. But nature is a wonderful thing and I think the bushes have come through it fantastically well."

Hinchliff was born 51 years ago in a beautiful farmhouse overlooking the Kent countryside towards Canterbury cathedral in the distance, and still lives there today. He has been growing blackcurrants for 25 years. He introduced them to his family farm – "because as long as the quality is good, you are guaranteed to sell them". Every July and August in the UK, blackcurrants are harvested using machines that drive up and down the rows, hugging the bushes and shaking the fruit from the branches. The berries are almost certainly destined to be made into one thing – Ribena blackcurrant drinks. Around 90% of all Britain's blackcurrants are sold to the company.

In its marketing and on its bottles, Ribena likes to stress that every berry is grown by British (and a few Irish) farmers: over the last six weeks, 39 farms have been harvesting around 12,000 tonnes of fruit, which are sent to a squashing factory in Somerset. From there, the juice is taken to Ribena's factory in Gloucestershire, where it is stored in huge underground vats, processed and bottled throughout the year. Named after the plant's Latin name, Ribes nigrum, Ribena was developed by a Bristol-based food and drink company, HW Carter, and launched in 1938. Because blackcurrants have a high vitamin C content, free Ribena was given to children during the second world war when fruit such as oranges became hard to get. Despite the fact Ribena is now owned by the drugs giant GlaxoSmithKline, people still have an affection for it, often rooted in childhood memories.

However, Ribena appears to be on a bit of a PR push. According to research analysts Nielsen, sales fell 2.4% last year and 6.8% in 2008, and the brand has faced criticism because of its high sugar content and the revelation, in 2007, that two schoolgirls in New Zealand had tested its drinks and found it didn't contain the level of vitamin C that the company claimed, although GlaxoSmithKline always maintained that the claims made about the level in drinks sold in the UK were accurate. And for all its focus on British-grown berries, a glass of diluted cordial contains just 5% fruit juice (along with a hefty dose of sugar). Perhaps mindful of this, it recently brought out a range of drinks with 85% fruit juice (from concentrate), but the majority of the juice is from apples, not blackcurrants.

Still, the livelihoods of British blackcurrant farmers and their employees depend on GlaxoSmith-Kline, which obviously gives the company a certain power. Hinchliff says it does not abuse that when it comes to pricing, and it puts money into developing plants that can withstand warmer temperatures, so farmers should be able to continue to grow crops in the future, as climate change takes effect.

Hinchliff grows four varieties. "Each has its own characteristics," says Hinchliff. I wondered if Ribena tastes different from year to year, as wine does, but I'm told it doesn't. "Some varieties are early, some are late," he says. "The blackcurrant farmers couldn't harvest everything at the same time, so you end up staggering the season and spreading the risk that you won't get a good crop. It's a great success story for British agriculture."

In the next field, which is protected by a soaring windbreaker of ash, oak and cherry trees, the plants are around nine years old and would reach above my head were they not bowing under the weight of ripe fruit; once the berries are picked, they will spring back upright. "This is a great crop," says Hinchliff, hauling back a branch laden with black pearls. The berries are warm and dusty, and yield easily with a pop. "That's what I have spent 12 months trying to get."