Kate Middleton's 'non-negotiable' with William and issue 'weighing heavily' on marriage

The Prince and have faced a brutal 12 months that would have rocked the foundations of any solid marriage. But with last year and gradually returning to royal duties, things are certainly looking much brighter for her and husband , who has been her constant support.

Next week they - 14 years of marriage with a two-day trip to the stunning Isle of Mull, where they will stay in a self-catering cottage and carry out a string of engagements. In recent years, they've also had to deal with the fallout of and Meghan dramatically quitting their royal roles as well as getting to grips with their increasing royal duties as Prince and following the death of the late Queen.

They also have to finely balance this with raising their three children , and - and being hands-on parents doing the school run and ferrying the youngsters around to their various activities and after-school sports clubs.

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Former royal correspondent Jennie Bond has delved inside their marriage to work out exactly what it is that makes their union a special one. And she exclusively reveals exactly what has allowed them to navigate stormy waters - and now appear to be even stronger than ever.

She explains: "Their first anniversary after Catherine’s official remission has to be that extra bit special. Every milestone takes them that little bit further away from the dreadful day when she was diagnosed with cancer. After his mother died, William said he knew it would make or break him and he refused to let it break him.

"I think they have both taken a similar attitude to dealing with the brutal year they have just endured. Their marriage is, I think, even stronger than before, and it was already a roaring success. I think they like each other. A lot. And that’s a whole different thing to loving one another -which, incidentally, they also do. A lot.

"What makes it work? I think theirs is a pretty equal partnership. William is a much more hands-on dad than his father or grandfather. And I don’t think Catherine would let him get away with anything less.

"They operate as a partnership and they have such a deep understanding of one another that they can communicate when they are out and about with just a look or a gesture. They seem sometimes to have private jokes that make them smile or nudge one another.

"I do think that Catherine’s parents and the King must be delighted that the marriage has worked out so well. It’s important for the monarchy that it works, but, on a human level, it is so comforting for the Middletons and for the King to know that William and Catherine are such a strong unit and seem able to deal with pretty much anything that life throws at them."

The couple's love story stretches more than 14 years, right back to 2001 when they met as students at the University of St Andrews in . As freshers, both lived on campus in St Salvator’s hall of residence, before going on to share a two-storey house with friends. And it was during this time that the pair fell for one another with the slow burn of William and Kate's early romance credited for being the key to their longevity.

However, it hasn't always been plain sailing for the couple. In 2007, after they graduated from university, they briefly split as William entered Army training and Kate started work as a buyer at fashion chain Jigsaw and they figured out what they wanted from life.

But their time apart was short-lived as just months after it emerged they were no longer together, they rekindled their romance with some believing the split reinforced their true feelings for each other. At the time of their engagement, even Kate confided that while she wasn’t very happy about it", she believes the experience made her a "stronger person".

"You know, over the years William has looked after me, he’s treated me very well – as the loving boyfriend he is, he is very supportive of me through the good times and through the bad times," Kate later remarked.

After marrying in a grand wedding at Westminster Abbey, Kate began her life of royal duties consisting of high-profile public engagements, prestigious state events and at times, gruelling overseas tours. In the early days of their marriage, the couple kept public displays of affection to a minimum, despite Kate once remarking how "hugs were important". But fast-forward 14 years, joint outings often consist of tender touches, loving glances at each other and even sweet hand-holding.

Last year, the couple laid bare their affectionate nature in a highly personal video to announce the end of Kate's cancer treatment, which saw William tenderly embracing his wife in a cuddle on the beach and placing a kiss on her. Earlier this year, the pair also posted a clip on their own social media accounts of them walking hand-in-hand through a railway station ready to catch the train home after a busy day of engagements in Wales.

"I think this gives us all the chance to see first-hand that theirs is a true love match, built on the strongest of all foundations: friendship and the shared experience of university life," says Jennie.

"Cancer must make you realise that life is short and is for living and loving. If they feel like touching or holding hands when they are in public, then why the hell not? I think everyone loves to see how strong their marriage is, and even more so after the trauma of the past year or so."

Another thing they can also count on, especially during their public life, is their shared cheeky sense of humour and competitive nature. Kate is often seen laughing at her husband's witty quips with the pair regularly heard gently teasing each other.

At the recent rugby match between Wales and England in Cardiff, which the royal couple attended, their mischievous facial expressions were on show for all to see as they cheered on their opposing teams.

The actual day of their 14th wedding anniversary on Tuesday falls on a school day so any celebrations William and Kate have planned will have to fit in around the school run, after-school clubs and homework. But even if celebrations are muted, Jennie predicts Kate could still be in store for a special gift.

She explains: "Traditionally ivory was the gift for the 14th wedding anniversary but obviously that’s illegal these days. Instead, people sometimes take an elephant theme - and that would be very much in keeping with William’s conservation work with the elephant trust Tusk. And, since ivory was banned, people have also chosen gold to replace it on the 14th anniversary. So maybe a beautiful golden elephant necklace or pair of elephant earrings might do the job!"

As their marriage continues, there will no doubt be more hurdles in the future for them to face. In the short term, they have a big joint decision to make over where their eldest child and heir to the throne Prince George should go to school once he turns 13 next year.

Their other pressing responsibilities include their charitable work with the causes close to their hearts, such as William's ambitious campaign to end homelessness with his Homewards project and Kate's desire to highlight the importance of a child's early years - not to mention William's growing expectations as heir to the throne. Just yesterday, he attended the funeral of Pope Francis on behalf of his father.

"It’s all very well thinking that Catherine has hit the jackpot by marrying a prince and future king", explains Jennie. "But becoming a senior royal and one day Queen is a truly extraordinary role to take on and could be a lonely and bewildering one. William, I think, has helped her grow into the role, and I think he is forever grateful that she has been willing to take it on and to give him three gorgeous children whom they both adore and the security of a happy family life which he, sadly, never knew."

However, another huge test will come with their ultimate destiny - becoming King and Queen. All of his life, Willliam has known that the role as head of state will be in store for him, and 14 years ago on their wedding day, Kate's future life was also sealed.

The cancer diagnosis of William's father the King and his ongoing treatment will no doubt also play on their minds and what this could mean for the future. But Jennie adds: "Together they are facing the challenge of becoming King and Queen, possibly rather earlier than anticipated, given the King’s cancer. That must weigh heavily on them, but, together, they are strong."