Seventy-seven per cent of coffee chain customers are drinking their purchase in the outlet. Those who spend 15 minutes in a coffee chain spend £4.78 per trip on average. However those who spend more than four times longer - at least an hour - only spend 10% more or £5.22 on average.

So how can customers who spend four times longer not spend, materially, any more money? Are they taking the Michael? Or there is another theory?

Many customers in coffee shops are working on laptops or taking a break during shopping, for example. Inevitably they will have valuables with them - a laptop, handbag, briefcase, or bags of shopping. And it is human nature that customers do not want to leave these valuables, while they queue up once again to order another drink or something to eat.

Our national psyche is often that Brits feel guilty about only buying the one drink in the coffee shop and most would be likely to buy another cup or perhaps something to eat - but they are not going to risk having their valuables stolen to get up and order. This is where staff can really help to drive sales during quiet times of the day - by simply offering table service.

There is also a huge opportunity to upsell other food and bakery items to customers. Of those buying either a hot or cold drink only half are buying food items. So they might start feeling hungry if they have been sitting down for an hour. Staff should not only ask if customers would like another drink, but also say, for example, "Would you like try some of our delicious banana cake with your coffee?" Or, more simply, coffee chains could offer lockers for people to store their belongings, so they can relax without worrying.