“We thought we were doing it wrong. We thought we must have made a mistake,” Anna told New Zealand’s Weekend Herald. The girls were both 14 and students at Pakuranga College in Auckland when they did the experiment in 2004. Given Ribena’s advertising claims that “the blackcurrants in Ribena have four times the vitamin C of oranges”, they were astonished and wrote to the manufacturers, GlaxoSmithKline (GSK). When they got no response, they phoned the company, but were given short shrift. “They didn’t even really answer our questions. They just said it’s the blackcurrants that have it, then they hung up,” Jenny said.

But then the girls’ claims were picked up by a TV consumer affairs programme, Fair Go, which suggested they take their findings to the commerce commission, a government watchdog. GSK said the girls had tested the wrong product, and it was concentrated syrup which had four times the vitamin C of oranges. But when the commerce commission investigated, it found that although blackcurrants have more vitamin C than oranges, the same was not true of Ribena. It also said ready-to-drink Ribena contained no detectable level of vitamin C.

GSK is in court in Auckland today facing 15 charges relating to misleading advertising, risking fines of up to NZ$3m (£1.1m). In Australia, GSK has admitted that its claims about Ribena may have misled consumers.