RICHARD KAY: Welcome to the most royal non-royal wedding of the decade: How did the £10bn Duke of Westminster – who marries today – pull off a feat of diplomacy by keeping William and Harry onside… even though only one will attend?

At noon today the bells of Chester’s medieval cathedral will ring out in joyous peal for a wedding.

For ambitious mothers of aristocratic daughters they may have a mournful tone, however. Their sound will mark the moment when Britain’s most eligible suitor will be struck from the list of the kingdom’s bachelors.

Fittingly, Prince William, who once himself headed that eminent roll call, will be a witness to the moment when the handsome, dashing and very rich Hugh Grosvenor, the seventh Duke of Westminster, marries his middle-class fiancee Olivia Henson.

History is full of occasions when an attractive young beauty courted by a man of fortune and title says ‘I do’. But rarely have a bride and groom had to navigate the perils of a social minefield in the weeks and months leading up almost to the moment when they exchange vows.

The reason, of course, is the Duke’s relationship with two royal Princes, William and Harry. Uniquely among that gilded company who once counted themselves as friends of both brothers, Hughie Grosvenor, as he democratically likes to be known, is something of an outlier.

The Westminster family home: Eaton Hall rises from the ground like a vast symbol of wealthy ostentation and the power of self-promotion

Where many chose sides — or had sides chosen for them — the Duke has retained his friendship with them both. He is godfather to their elder children, Prince George and Prince Archie, but the strain of such divided loyalties was threatening to overshadow the big day.

It took the last-minute intervention of People, the American feelgood magazine which has close links with the California-based Duke and Duchess of Sussex, to explain that Harry had declined his invitation because of the challenges his presence would cause.

Translation: his fraught relationship with William.

Whatever the truth of this noble gesture, it has avoided the potentially awkward situation of the Prince of Wales, an usher at today’s event, showing his estranged brother to his seat.

All the same, questions about who was and who wasn’t invited are likely to be as intoxicating as the vintage champagne due to be served at the reception. Even without Harry and the absence of the King, who is the groom’s godfather, today still promises to be the most royal non-royal wedding of the year — if not the decade.

Given the circumstances — the duke’s £10 billion fortune makes him at 33 the richest person under 40 in the land — it could be the most lavish and over-the-top affair. A few miles from the cathedral stands the Westminster family home. Eaton Hall rises from the ground like a vast symbol of wealthy ostentation and the power of self-promotion.

Once, it was a gloomy Victorian pile only to be bulldozed and replaced by a brutally modern concrete construction. Three decades ago, Hugh’s father oversaw another facelift, giving the property the elan of a French chateau.

Chester Cathedral where the couple will be married. The cathedral is only a few miles from the Westminster family home Eaton Hall

A fleet of florists’ vans delivered scores of boxes of white and purple foxgloves and wild flowers to decorate the 1,000-year-old cathedral

… as a towering stack of chairs is carried by a workman into the venue

… and spectacular blooms in hues of purple and white arrive by truck 

This arrangement was so huge it took a stepladder to make the final adjustments

In the neat villages that make up the estate, lawns were being mowed and hedges clipped as the duke’s tenants made their own contribution to the wedding day. Some will rub shoulders with the 400 aristocrats and other society figures invited. But those expecting an exercise in extravagance may be disappointed.

‘The duke’s a very unassuming young man,’ says one estate figure. ‘He doesn’t seek out the limelight, which is the way we like it too.’

And it seems his bride is cut from the same self-effacing cloth. The daughter of a stockbroker, she was educated at Marlborough, the Princess of Wales’s old school, and until recently was working as account manager for an ethical food company.

But low-key doesn’t mean they haven’t put a lot of thought into the wedding. They are eschewing the traditional English service used by other royal couples for something a little more modern.

‘It reflects the fact that they are a contemporary couple,’ says the cathedral dean the Very Rev Dr Tim Stratford. He says they have made specific choices for the scriptures and hymns that have meaning to them.

‘In many ways, this is like any other couple getting married, just on a larger scale, because they are somewhat in the public eye and some of their guests are definitely in the public eye,’ the dean says.

‘The couple at the centre of this want to declare their undying love for one another in the presence of witnesses, family and friends and we are here to provide the stage.’

Meanwhile, lavish preparations were under way. A fleet of florists’ vans delivered scores of boxes of white and purple foxgloves and wild flowers to decorate the 1,000-year-old cathedral. The florists, from Flower And Press, who provided displays for actress Lily James’s 30th birthday, like to boast their sustainable credentials — many of the flowers used will be dried, pressed and recycled into artwork.

Given the presence of the heir to the throne Prince William at the event, security precautions were also in evidence, with police sniffer dogs checking drains and manholes

In the neat villages that make up the estate, lawns were being mowed and hedges clipped as the duke’s tenants made their own contribution to the wedding day

A giant arch was constructed around the huge West Door to the building, decorated with branches of oak and other trees — reminiscent of the display of foliage at William and Kate’s Westminster Abbey wedding. Teams of riggers from Starlight Design, an edgy lighting company, were setting up purple and blue spotlights in the cathedral, while chairs wrapped in plastic were being unloaded.

Given the presence of the heir to the throne at the event, security precautions were also in evidence, with police sniffer dogs checking drains and manholes.

Chester, it has to be said, was containing its excitement, though on a road into the city near the entrance to the Grosvenor Hospital, staff had dressed a pair of mannequins in suit and wedding dress with photos of Hugh and Olivia’s faces attached.

Details about the guest list and reception plans were being kept private, at least for now, but the Duke has announced that the couple will be treating the city’s citizens to free ice cream. One parlour is also offering lemon-flavoured Arctic roll, a nod to the couple’s lemon wedding cake.

It’s not quite money behind the bar but as one local said, ‘It’s the thought that counts.’

Grosvenor is also paying for this year’s Summer Flowers, the annual blooms that are planted round by the city by the council.

There was just time for one more essential element last night: the wedding rehearsal. The Duke was said to look ‘pale and nervous’ by onlookers but Olivia, 31, in a blue patterned dress and trendy red clogs, was described as ‘relaxed and radiant’.

A group of residents applauded as the couple, together with 25 or so supporters including the Duke’s brother-in-law, the historian Dan Snow — married to his sister Lady Edwina — emerged after half an hour. Olivia smiled back at them and seemed to be brimming with excitement as she hugged a member of her party.

‘It’s all very exciting,’ said one local. ‘Now we’re hoping to see Prince William tomorrow.’

Should he have time, William might pop into the cathedral shop: among its bestsellers are tea caddies bearing the face of his late mother, Princess Diana, a former Countess of Chester, after whom the hospital is named.

A generation ago, amid the bitter feuding of Charles and Diana, the Duke’s parents showed the same remarkably even-handed approach to the warring Prince and Princess, remaining friends with them both. It’s a valuable lesson that has clearly been learned by the soon-to-be-wed Hugh Grosvenor.