Etiquette-chup: can tomato sauce ever be part of a posh dining experience?

Ketchup is the Ed Sheeran of the condiment world: staggeringly popular with everyone from kids to grannies, but neither fashionable, exotic, nor particularly sophisticated. Sweet, brightly coloured and proudly middle-of-the-road, it's just there, and it's everywhere.

But unlike Ed Sheeran, ketchup has had a tough couple of years. In 2017 it was reported that a dip in sales saw fancy European interloper mayonnaise overtake red sauce as the UK's favourite. After a century or so as Britain and America's best-loved flavour-enhancer, were we finally starting to fall out of love with the tangy tomato treat?

Perhaps, which explains why ketchup has been given a much-needed publicity boost – with none other than Sheeran himself set to star in an upcoming Heinz advert.

Reports this week suggest the campaign will feature the Suffolk songsmith shocking staff at a five-star hotel in London by whipping out a bottle of ketchup as he sits down to a posh meal. Apparently, the ad is based on real life, with an insider saying: "Ed has experienced firsthand how people can turn their nose up or scoff at asking for ketchup in posh restaurants. He wants to poke fun at how ridiculous that is."

It is said that one of Ed's assistants is always on hand with a bottle, lest a high-end restaurant decline his request. Sheeran even has a tattoo of a ketchup bottle on his arm, and in 2017 the red-sauce superfan announced his manifesto, should he ever become prime minister: "Ketchup compulsory in all places with chips."

The singer's simple foodie predilections will come as no surprise to his fans. Last year, Sheeran told Jessie Ware's Table Manners podcast he was once known as "three puddings Ed", and has previously stated: "why have abs when you can have kebabs." Presumably smothered in ketchup, which as everyone knows is one of the worst ways to eat kebabs.

But do fancy restaurants actually mind if a customer asks for a side order of ketchup? Is it a customer's prerogative, or the fastest way to infuriate a chef? Comedian Mickey Flanagan thinks both. In his famous routine about a visit to a fancy restaurant with his "middle-class" wife, he encounters the "awkward situation" of ordering ketchup. His wife pleads him not to. "I've gotta have the tomato sauce... I've just ordered the risotto," he tells her. Inevitably, "two chipfuls" are delivered to the table; effectively a middle finger from the chef.

At many establishments, chefs spend hours, days and weeks perfecting flavour and texture combinations. Surely it's a little disheartening to then witness a diner squirt Heinz all over it, turning a work of art into a crime scene.

"Ketchup is only available with our cooked breakfast," says Carla ter Maat of Drakes in Brighton. "It would indeed be insulting to the culinary skills and the trained palates of our expert chefs should a guest request a bottle of ketchup at dinner. Adding ketchup would be tantamount to a slap in the chef's face."

On the other hand, isn't the customer always right?

"Taste is subjective," says Jonathan Dale, a former Masterchef contestant, "but adding ketchup to a dish that has been crafted and balanced by a chef is odd. I personally think it shows poor judgement and is kind of disrespectful. If that's the taste you want, why bother with 'fancy' food? Each to their own, though."

Tommy Heaney, chef proprietor of Heaney's Cardiff, agrees. "We never say to customers 'this is how we've made the dish so you have to eat it like this'. I wouldn't personally eat ketchup with many things, but one of the best things about cooking is that everybody's taste is different. And at the end of the day, the customer is paying the bill."

Jordan Kelly-Linden, food writer and Telegraph video producer says the acceptability of requesting ketchup "definitely depends on the cuisine and style of food. If fried potato is in any way involved, yes. If you're trying to stick it on some pasta or seafood, you're a wrong'un."

A wrong'un, or a child. Ketchup is still regarded in many circles as a juvenile choice, something for kids to pour over any meal they'd otherwise find unappetising. Whether at a restaurant or a friend's dinner party, asking for it can leave you feeling a bit immature (unlike, say, a request for wholegrain mustard or horseradish).

It shouldn't be so, says luxury lifestyle expert Paul Russell. "A restaurant that looks down their nose at you for asking for a particular condiment is pretentious and snobby, and that is not fine dining.

"Having said that, I can't think of any fine dining dish that would benefit from the addition of ketchup."