There is a fine memorial in Green Park in London to the pilots and crews who served with Bomber Command during the Second World War.

There is a stunning memorial at Capel-Le-Ferne near Folkestone to the Battle of Britain pilots.

What there is not is a national memorial to the crews of the Photographic Reconnaissance Unit.

First formed in 1939, they flew critical flights over enemy territory in planes especially adapted to carry precision cameras to photograph and identify enemy targets, both before bombing raids and afterwards to assess the damage.

From October 1942, the unit was split into five squadrons: Nos: 540, 541, 542, 543 and 544.

The unit flew a variety of aircraft from Spitfires, Bristol Blenheims and Lockheed Hudsons to de Havilland Mosquitos.

During the six years of war, the unit took more than 20 million photographs and the intelligence they provided was crucial to the success of the Dambusters raid at the Mohne, Eder and Sorpe dams, to the finding and destruction of the Tirpitz battleship, and to the discovery of the German's V1 and V2 weapon production sites at Peenemunde in Holland, to name but a few.

For Guy Gibson and his courageous Dambusters, Barnes Wallis had calculated that his bouncing bomb would work only when the water level in the reservoir was exactly 4ft from the top of the dam.

Gibson later wrote: "All the time we were training, we had reconnaissance aircraft out flying over Germany watching the dams as a cat watches a mouse. They never flew straight to them because then the Germans would know what they were looking for, but always took a roundabout route crossing the dams as if by accident. The first and most important thing they were looking for was the height of the water level - it was slowly rising."

The airmen of the PRU were able to pinpoint exactly when the water level was just right.

Their work meant they were constantly flying unprotected over enemy territory, risking attack from enemy fighters and flak.

Without their courage, Bomber Command would have been flying their missions blind.

But it came at a cost: more than 500 PRU airmen lost their lives from an estimated total of 1,300 who flew operational sorties - one of the highest casualty rates of any unit.

Among them was John Harrington Loder from Lenham.

The son of John Edmund and Gladys Mary Loder, his Mosquito was shot down in 1943. He was 25.

He is buried at The Bouogne Eastern Cemetery at Pas de Calais in France.

Flt Lt Loder of the Royal Air Force Volunteer Reserve had already been awarded the DFC for bravery almost two years earlier, in September 1941.

Attached to 544 Sqn, he and his Canadian navigator P/O Trevor Hughes were assigned a series of night photographic missions – dropping high intensity flares from altitude to illuminate the ground to catch night movements of troops and supplies over northern France.

The pair had carried out successful night missions previously, but on May 15, they took off from RAF Benson at 10.25pm and were due back around midnight, but their plane was never seen again.

Sometime later, Flt Lt Loder's body washed up on the shore of the English Channel near Boulogne-sur-Mer, suggesting they crashed in the sea. The navigator's body was never found.

John Loder is remembered on the Lenham War Memorial and has a street in the village named after him.

But now a national campaign is under way to provide a fitting memorial to the members of the Photographic Reconnaissance Unit.

It is named the Spitfire AA810 Project, after one particular aircraft that flew with the unit.

The Spitfire AA810 was one of the fourth development of Spitfire adaptations for photo reconnaissance work - the Spitfire P IV.

They were nicknamed the "bowsers" because of their impressive fuel-carrying capacity.

On top of the standard 85 gallon fuselage tanks, the PR IV carried an additional 66 gallons per side in leading edge fuel tanks, extending the range from 575 miles 2,000 miles.

The downside was that the aircraft had to be stripped of all guns and armour plate, to compensate for the extra weight of fuel, leaving it utterly defenceless.

Perhaps not surprisingly Spitfire AA810 was shot down on just her 16th mission, while photographing the Tirpitz.

Helen Whately, the MP for Faversham and Mid Kent, is among several dozen MPs who are supporting the campaign for a national memorial.

She said: "The intelligence provided by the PRU was instrumental in the planning of many major operations.

"We owe so many people - including our local hero John Loder - a debt of gratitude.

"Heroes like John deserve to have their service recognised and commemorated which is why I’m supporting plans for a memorial.”

Further details of the campaign can be found here.

The campaign is keen to hear from relatives of other flyers who served with the PRU. They can contact Tony Hoskins by email to Tony@spitfireaa810.co.uk.