“We’re London’s best kept secret”, crew members at London’s Air Ambulance’s Whitechapel base tell me as we meet up just before the aircraft’s 25,000th mission.

The charity celebrated that milestone at the end of last month and, while its stellar reputation makes it likely it will raise the cash needed for another quarter of a million flights, the team are probably right.

“Not many people know we are a charity. We are a small operation but doctors come from all over the world for placements because they just can’t get this sort of experience elsewhere,” Chris Sutton, one of LAA’s four aviation firemen said.

While the work LAA does on a day-to-day basis may go under the radar, it has an elevated status among helicopter medical teams around the world.

In its 23-year history, it has made a name for itself as a pioneer of life-saving roadside treatment and is also recognised for its role in major catastrophes like 7/7, the Canary Wharf bomb and the Potters Bar train crash.

Mr Sutton added: “During 7/7, the air ambulance came into its own as a means of transport. We deal with trauma cases and our main function is to deliver a medical team as quickly as possible.”

LAA played such a vital part in ferrying doctors to and from the bomb sites that Lady Justice Hallett, coroner in the 7/7 inquest, said it deserved “proper funding and proper recognition” during a hearing for the on-going investigation last month.

A combination of NHS money and public donations keeps it afloat.

As London’s only air ambulance, LAA deals with the most serious of the 4,000 daily call-outs London Ambulance Service receives.

Covering any area within the M25 for a population of 10 million on any given day is no mean feat.

The helicopter can reach any part of Greater London within 12 minutes of a call-out.

In the control room, the on-duty paramedic is trained to decide which incidents the helicopter or rapid response car should respond to.

Criteria they look out for include falls from more than four storeys, shootings or a road crash where a victim is trapped in a vehicle.

Each mission is made up of a pilot and co-pilot, one senior trauma doctor and one paramedic.

Doctors are signed up for six-month placements and come from as far afield as New Zealand, America and Europe.

During my visit to LAA’s air base on top of The Royal London hospital, anaesthetist Doctor Vidar Magnusson was almost two-months into his placement.

The Icelandic doctor had been working in Norway, which has its own highly-esteemed air ambulance service.

But the challenges facing LAA are greater, he said, because of the seriousness of the cases and the sheer size of the population.

He explained: “The way London’s Air Ambulance is organised is second to none. It’s a very tightly-run ship.

“Some patients that LAA attends to needing open heart surgery would otherwise be declared clinically dead. It is able to save 15 to 20 per cent of them – that’s pretty impressive.”

Alongside the 25,000th mission, LAA is this month celebrating its first year of being a 24-hour operation.

After the helicopter signs off the day, the rapid response cars take over for the night shift.

Earlier this year, LAA agreed a deal with banking giant Santander to lease the helicopter for seven years.

Speaking of how important the service is, LAA’s medical director and chair, Dr Gareth Davies, said it “saves the lives of many who may not have otherwise survived the distance to hospital.”

He added: “I am extremely proud of the team. London is a first class city and we have always strived to deliver first class care.”