Leonard said: “I felt a little bit in awe. I was taking in the devastation it could create.
“I was 20 years old then and not quite sure what was happening.”
It was 1956 and Leonard had just witnessed Operation Mosaic, part of the British nuclear test programme of the 1950s and 60s.
He was standing just 14 miles away from the site of the explosion.
Leonard was one of around 22,000 British service personnel who witnessed nuclear tests on mainland Australia, the Montebello Islands off Western Australia and Christmas Island in the South Pacific, during the period.
The men were usually issued with little, if any, protective equipment, exposing them to high levels of radiation.
Many went on to suffer ill health, sometimes many years after the tests, with an increased rate of cancers, infertility and birth defects observed in veterans and their descendants.
The British programme began with Operation Hurricane on October 3 1952 and culminated in Grapple, a set of four nuclear weapons test series of early atomic bombs and hydrogen bombs carried out in 1957 and 1958 at Malden Island and Christmas Island.
This included Grapple Y, which remains the largest British nuclear weapon ever tested, dwarfing the bombs that were dropped on Hiroshima and Nagasaki at the end of World War II.
Now two local veterans have spoken out about their personal experiences of the tests.
Leonard was on the HMS Alert when it arrived at the Montebello Islands in March 1956. In May, he bore witness to Operation Mosaic.
Leonard, who lives in Rowan Close in St Leonards, said: “On the day of the test we went to the upper deck. We were wearing just shorts and flip flops.
“The only information they told us was to turn our backs and close our eyes, so we did.
“The explosion happened. We felt the heat on our backs and we were told to turn around and face the explosion.
“We saw a bomb mushroom cloud in the sky.”
Leonard said: “I was lucky being on HMS Alert.
“Members of the Army were in trenches at various distances from the explosions.”
In May the following year, Leonard and the rest of HMS Alert arrived at Christmas Island.
Leonard was present for the detonation of two hydrogen bombs at the beginning of the Grapple series of tests.
He said: “For this one they gave us what looked like boiler suits to wear. But we still had to turn out backs to the explosions.
“I also had a radiation detection badge which was pinned to us.
“There was definitely heat on our backs, despite wearing a boiler suit.”
Leonard suffered no ill health as a result of his presence at the tests, but discovered the fate of many of his fellow servicemen via the magazine produced by the British Nuclear Test Veterans Association (BNTVA).
He added: “The stories that have been in the magazine have been quite alarming.”
Bexhill nuclear test veteran David Williams had an unusual health scare which he believes was linked to his involvement with the nuclear testing programme.
David, of Ocklynge Close, was an aircraft engineer in the RAF.
He was stationed at Christmas Island for 12 months from January 1958.
During that time he witnessed the detonation of three H-bombs and two A-bombs as part of Operation Grapple.
As a young man of just 20, David, like many of his fellow servicemen, was kept in the dark by the Ministry of Defence and was unaware of the gravity of the tests at the time.
He told the Observer: “On the day of the tests, you got up at four in the morning and we went to the mess then we would all drive down to the port.
“We were lined up like sheep in various sections then they said over the loud speaker the aircraft was in the sky.”
He added: “When it came to dropping the bomb itself, they said to sit down, turn our backs away from the aircraft and then they told us the weapon had left the aircraft.”
Twenty seconds before the detonation, the men were told to cover their eyes with their hands.
David said: “You felt the blast on the back of you. It bent the palm trees over.
“Then they said turn around and have a look and you could see the outline coming into the mushroom. And that was it.”
Just like Leonard at Operation Mosaic, David was provided with no protective clothing.
David believes this exposure to radiation led to him developing a mysterious growth on his chest later in life.
He said: “About 11 years ago, I had a little growth on the front of my chest, like a little rhino horn. And it was hard.”
Two doctors told David the growth was harmless and he went into hospital to have it removed.
He said: “The hospital cut it open and they took it out.
“It was the shape and size of an egg and it was all jelly-like.”
David later received a phone call to tell him the lump was in fact a sarcoma, a rare form of cancer. Together all types of sarcoma represent just one percent of all new cancer cases.
David, now 82, said: “They had no idea what it was.”
Nearly 70 years after the first nuclear test, Leonard, David and their fellow veterans have yet to receive any recompense or recognition from the British Government.
This is in stark contrast to other nations.
Leonard said: “The annoying thing is that New Zealand, America and Australia reward their veterans medals and recognition.
“But our government just fobs it off.”
This week the BNTVA submitted a 90-page medal application to the government.
Meanwhile the charity continues to promote research into the effects of radiation, in the hope that one day it will be recognised that veterans were harmed by the tests.
But Leonard said: “I’m 84 now. By the time they make their minds up, us veterans will be well and truly gone.”