The Duke and Duchess of Cambridge opened a new £18m children's cancer unit at the Royal Marsden Hospital in Sutton yesterday.

Young cancer sufferers, their parents and dedicated medical staff met the royal couple, both 29, as they were shown around the Oak Centre for Children and Young People.

William is president of the Royal Marsden, a role he inherited from his mother, while Kate is deciding which charities to support as she shapes her future role.

The Duke unveiled a plaque to officially open the centre before he and the Duchess also met key donors and fundraisers. During the visit the William was reunited with Alice Marples, a brain tumour patient who interviewed him two years ago for the BBC Newsround programme.

The 12-year-old presented Kate with a bouquet of flowers after her husband William unveiled a plaque towards the end of their visit.

Alice, who has already had two 18-month sessions of chemotherapy at the Royal Marsden, travels to the United States on Monday for pioneering Proton therapy.

She said: "It was such an honour and privilege to see them again. I'm going to Oklahoma next week for 8-10 weeks so it was lovely to see them again."

Staff and children from the Playhouse Day Nursery, based at the Royal Marsden, waved flags and banners as Duke and Duchess arrived.

Some of the nursery's 42 children wore crowns in tribute to the couple and William told one three-year-old, called Finlay, that his headwear looked "Awesome".

Search and rescue pilot William earlier shrugged off a sleepless night after two call-outs, one rescuing a woman who suffered head in jury following a fall in Snowdonia.

He then had to fly a sailor to see a seriously ill relative, ferrying the serviceman from a warship off the Isle of Man to Sandhurst in Surrey before arriving back at RAF Valley at around 9am.

Referring to his overnight search and rescue call-out, William joked: "It was a bit of an early morning. It's great to be here finally - we've been talking about this for a while."

Kate wore a simple, pale knee-length dress with elbow length sleeves for the engagement - teamed with LK Bennett nude Sledge heels and a Natalie clutch bag - while her husband arrived in a navy suit with a pale blue shirt and burgundy tie.

Their Royal Highnesses spent more than two hours talking with children and insisted on seeing every patient in the ward.

Unveiling a plaque inside the new ward, the Duke said: "This place is inspirational and it always inspires me. I always come away from the Royal Marsden completely uplifted and Catherine and I will do the same today.

"Once again the Royal Marsden is pushing the boundaries of cancer treatment and research. As president of the Royal Marsden I thank you all for making this a reality.

"It gives Catherine and I great pleasure to open this fantastic facility."

William's mother Diana opened the children's unit at the Royal Marsden in 1992 and, like her, is a huge supporter of the hospital..

The Princess auctioned 79 of her dresses to raise funds for the hospital, at her eldest son's suggestion, only weeks before her death in 1997.

In December 2005, William undertook two days work experience at the hospital, working in the Children’s Unit.

He helped out in a variety of departments, including medical research, catering and fund-raising, as well as visiting a number of wards.

The Oak Centre has 31 beds and is funded entirely from charitable donations, including £3.5m from the Teenage Cancer Trust.

It will see almost 600 in-patients and more than 5,000 day patients per year.

Cally Palmer, Royal Marsden chief executive, said: "The centre is named after the Oak Foundation which generously donated £10m towards the cost of the building.

"Our new centre is one of the largest of its kind in Europe and we are incredibly fortunate to have had the support of our president."

Founded in 1851, The Royal Marsden was the first hospital in the world dedicated to cancer treatment and research into the causes of cancer.

There are two hospitals: one in Chelsea, London and the other in Sutton.

They also have a Chemotherapy Medical Daycare Unit at Kingston Hospital and provide Sutton and Merton Community Services.