Kate MiddletonandPrince Williamare in the party mood!

The royal couple recently surprised guests at a celebration marking the 75th anniversary of the U.K’s publicly-funded National Health Service and put the finishing touches to a birthday cake with the help of formerGreat British Baking ShowfinalistAlice Fevronia.

“The prince’s piping skills were less good than the princess’s, shall we say,” Fevronia tells PEOPLE about Kate and William’s baking skills after they both helped her decorate some cupcakes — with Kate also adding a topper to the bigger cake.

“They were just really fun,” Fevronia adds. “They were just having a fun time.”

Mel Giedroyc and Prince William and Princess Kate.Tom Dymond/REX Shutterstock

Prince and Princess of Wales surprise NHS staff and patients with 75th anniversary party

Tom Dymond/REX Shutterstock

“I was walking in behind them and ended up standing next to this lady and she almost grabbed me and just looked at me, went, ‘oh my gosh!’ because she was so shocked.”

Prince and Princess of Wales surprise NHS staff and patients with 75th anniversary party

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During the event, the Prince and Princess of Wales met a number of patients and health workers as they helped prepare the party at the well-being garden of London’s St. Thomas’s Hospital in central London.

There was also time for them to have a light-hearted debate about whether cream or jelly should go on a scone first. For the record: Kate went for cream first, while for William it was simple — whatever is closest to him at the time, he said.

Prince and Princess of Wales surprise NHS staff and patients with 75th anniversary party

Along with marking the birthday of the NHS, Kate and William also attended the event to shine a light on NHS Charities Together for what it does to help those in the health service who need mental health and well-being support.

While the NHS provides free care at the point of need, there has always been some reliance on locally based charities to help fill gaps in the system and give further support to hospitals and medical centers.

Prince William and Catherine Princess of Wales and baker Alice Fevronia.Tom Dymond/REX Shutterstock

Prince and Princess of Wales surprise NHS staff and patients with 75th anniversary party

Fevronia herself has many family members who have worked in the NHS and her partner is a pediatric doctor, working long stressful hours. “[The royal couple] were asking. How does he decompress and how does he relax? And I think it’s making sure that there’s people to talk to because it’s such a busy job,” she tells PEOPLE.

Party host and former Bake Off presenter Mel Giedroyc, adds that “The NHS is part of our DNA in this country. It is so precious. I hope, for the next 750 years, will grow and be nurtured and be loved and properly taken care of."

Prince William with guests at the party for the NHS.Tom Dymond/REX Shutterstock

Prince and Princess of Wales surprise NHS staff and patients with 75th anniversary party

The Princess of Wales with guests at the party.Tom Dymond/REX Shutterstock

Prince and Princess of Wales surprise NHS staff and patients with 75th anniversary party

Thomas sat next to William for part of the tea. “I have always admired him – from when he was a teenager, growing up with their mother. And he is a fine figure of a man and as I held his hand I said ‘Do you realize how handsome you are?’ I think he said ‘I haven’t heard that before.’ I said ‘You could be the next James Bond!’ " she smiles.

“What a wonderful backup for the NHS to have somebody like that coming in to share stories with us. It is a wonderful support,” she adds.

Prince William enjoying a laugh with a guest at the NHS Big Tea party.Tom Dymond/REX Shutterstock

Prince and Princess of Wales surprise NHS staff and patients with 75th anniversary party

Thomas handed William a copy of her book about her historic birth and the life that followed calledHold On Edna!

“Kate came over and said she’d read it – and she gave me a hug,” she tells PEOPLE. “It was like one of my grandchildren. It was the icing on the cake.”

source: people.com