Over the period, "average prices have fallen in every category except soft fruit", he said, with potatoes faring worst of all, averaging a 13 per cent price drop. At the same time, all categories except bananas saw a rise in the volume of sales.
Within the trend of falling prices and rising volumes there have been "winners" such as squashes and cherries, which saw substantial volume gains against low price cuts, and "losers" such as broccoli and lettuce, sales of which grew less in volume despite larger price cuts, said Cowan.
"Tesco went from selling 7.1 million iceberg lettuces to 10.3 million, but they went from making £5.9m on them to £5.1m. The real concern is in year two, when consumers' expectations have adjusted (to the new lower prices)."
Within the tomato category, "you can exploit different channels", he added. "Retailers have succeeded in getting people online, where higher earners shop more. You can help retailers add value here," given that many high-value fruit and vegetable lines already significantly "over-index" online, he explained.
Vine tomatoes are now worth more than ordinary tomatoes, even though volumes remain lower, he pointed out, while more frequent buyers are more likely to choose specialist types over ordinary tomatoes. Cowan concluded that for tomatoes at least: "There isn't a clear interaction between price and sales volume."
As to how the UK industry might better promote itself, he said: "You should understand who you are targeting and what their barriers to consumption are. What does British-grown mean to them? Is it better tasting? The public doesn't like being shouted at on health - there's been five years of shouting about 5 a Day."
Tomatoes: Key facts on current state of the UK’s tomato market
The third most valuable fresh-produce category behind top fruit and potatoes, worth £698m, amounting to sales of 250,000 tonnes.
• Bought by households 24 times a year on average.
• Market penetration of 93 per cent of households.