Prince William, Duke of Cambridge, and Kate, Duchess of Cambridge, are welcomed by the Queen's representative Deputy Lieutenant Air Vice-Marshall Clive Evans

The Duke and Duchess of Cambridge were in south London this afternoon to open a new £18 million children’s cancer unit.

The Duchess of Cambridge arrives at the Royal Marsden Hospital in Sutton

William arrived after a sleepless night during which he flew a mercy mission to take a sailor to see a seriously-ill relative, ferrying the serviceman from a warship off the Isle of Man to Sandhurst in Surrey.

The royal flew back to his base at RAF Valley on Anglesey just after 9am before making the trip to open the Royal Marsden’s Oak Centre for Children and Young People in Sutton, a few miles from Sandhurst.

The Duke, an RAF search and rescue force helicopter co-pilot, carried out the “compassionate call-out” during a normal 24-hour shift which also saw him help pick up a casualty with serious head injuries in the Lake District.

Before William and Kate’s arrival, patients and hospital staff crowded on to the building’s balconies, some waving Union flags.

Kate, Duchess of Cambridge, arrives at the Royal Mardsen Hospital in Sutton, south London

A huge cheer erupted as the royal car pulled up.

After waving to the crowds, the Duke and Duchess met Royal Marsden’s chief executive Cally Palmer and chairman Ian Molson.

Referring to his overnight call-out, William joked: “It was a bit of an early morning.”

He added: “It’s great to be here finally - we’ve been talking about this for a while.”

William and Kate, both 29, met patients and their parents during a tour of the centre, which has been built to treat day and inpatients.

The couple met two young leukaemia sufferers as they toured the new unit which has greatly improved the facilities for patients.

They joined Digby Davidson, 14, and his parents, Hermione, 41, and Fergus, 38, in a four-bed ward. As the Duke walked in, he had his jacket off, sleeves rolled up and was rubbing antiseptic gel into his hands.

Digby was treated for cancer last year but suffered a relapse which meant he had spent much of the previous three months in the new hospital unit which began treating patients in 2010.

William asked the teenager if they could join him on the bed and they laughed as the head of the bed was lowered and the royal couple sat either side of the young patient.

In a nearby bright lime green alcove containing a series of shelves, Digby had a few personal possessions and William quickly spotted a picture of the 14-year-old in military uniform.

“Is that you?” the Duke asked, and Digby, dressed in flip flops, bright Bermuda shorts and a dark T-shirt, explained he was in the Army Cadet Force.

Mrs Davidson made everyone laugh when she said her son had missed Army camp because of his relapse.

The teenager and his parents spent around 10 minutes chatting privately to the royal couple and Mrs Davidson said afterwards: “William was joking about Digby’s flip flops and shorts - he said he had a tropical theme that fitted in with decor.”

Later Harry Poil, 12, and his father Tony, 55, a Gatwick Airport security officer from Crawley, West Sussex, met the Duke and Duchess.

The schoolboy said sport was the topic of conversation he shared with William.

The 12-year-old said: “I’m a Chelsea supporter and we were chatting about last night’s (Champions League) match.

“We had a penalty against us and William thought it was a bit dubious.

“He asked me if I was a cricket fan, and I said I was, and if I had been following the Ashes series.

“Kate said she was more into rugby than football.”

William is president of the Royal Marsden NHS Foundation Trust - the first hospital in the world dedicated to cancer treatment and research into the causes of the disease.