FOULNESS Island is a well hidden treasure chest of Top Secrets. On the surface, this Ministry of Defence-owned countryside paradise boasts big blue skies, wide open spaces and a wildlife sanctuary for visiting birds, water voles, seals and badgers.

But the real business end of the island, which is strictly pass holding residents only, and shut off to the general public, has been all about testing military weapons for the best part of 100 years.

The MoD began its wholesale purchase of Foulness for this very reason back in 1911.

The Finch brothers owned the land and were at loggerheads over the sale. Unfortunately, the sibling who disagreed with turning the island into a controlled war zone died first, and the deadly deal was done.

The area was already considered an island way back in Roman times and the name seems to have derived from the old English, fulga-naess, meaning wild birds nest.

A rambling tour around the tranquil fields and moat-like, brown, water-filled ditches (called upside down hedges by the locals) of this nature haven is a surreal experience.

It is hard to believe the military are exploding grenades, rockets and shells here during office hours.

Foulness, surrounded by the rivers Crouch and Roach as well as the North Sea, could so easily be a designated area of natural beauty, or national park, full of walkers, cyclists and camping enthusiasts.

Maybe it is this uneasy alliance of fine balancing, between island man and military, which preserves Foulness’s best features so perfectly.

Maybe the wildlife and untarnished greenery would disappear forever if civilians were given access all areas to trample across this remarkable place, whose 156 permanent residents haven’t even been able to log on to the internet yet.

Barn owls certainly feel secure here. An ancient ramshackle granary resting beside a marshy pond is entered by one of my knowledgeable guides, and seconds later a pair of these snowy creatures slowly glide in a circle away from their home.

The peaceful illusion is instantly shattered on approaching the off limits North Sea wall of the island, as the harsh reality of Foulness’s true military role zooms into focus.

Crudely constructed brick watch towers, looking like something from a Cold War Lego kit, are dotted along the coastline.

From elevated, glass-fronted chambers, weapons testers get a clear view of the 30,000-acre Maplin Sands, which accommodate the MoD’s target practice.

Sitting on top of the towers are black wooden bird boxes.

“Killers with a conscience,” smiles one of my colleagues. Travelling parallel along the North Sea wall there is an area enclosed by a wire fence, housing old jets and big, round, cylinder rockets from a much bigger aircraft.

I’m told petrol head Richard Noble tested his record-breaking car here, Thrust2, which held the land speed record between 1983 and 1997.

Opposite the existing sea defence is another long strip of tough granite wall, with chunks blown out of it, revealing the twisted and burnt innards of a rusting steel skeleton.

This is a copy of Hitler’s Atlantic Wall, which stretched along the west coast of mainland Europe from the French/Spanish border, right up to Norway, designed to repel an invasion from Allied forces.

The wall was reconstructed on Foulness, so weapons could be tested against its durability in preparation for the D-Day landings, giving our boys a huge advantage as they successfully flocked across the sea to Normandy in 1944 to meet the Germans head on.

An opening in the sea wall leads on to Fisherman’s Head, a green sludge-covered, concrete wall built out from the coast into the sea to prevent the movement of the waves from removing parts of the land.

It’s wide enough for a car and extends across the grey, sticky porridge of the endless Maplin Sands.

Millions of white cockle shells and a handful of long-spent bulky ammunition casings hug the feet of the walkway.

A military vehicle straight out of the Action Man catalogue is parked at the side of this eerie tidal jumble, complete with caterpillar tracks and a crane arm.

Its duty is to retrieve the remains of weapons propelled into the sands.

Fisherman’s Head is also one end of the most bizarre public footpath. The Broomway is a six-mile stretch of hard sand, only accessible for four hours a day, when it is revealed by the low tide.

The Broomway crosses the treacherous sinking Maplin Sands, which has claimed a jeep foolishly trying to navigate its precarious path and the lives of the odd poacher or two, over the past few decades.

Its name is borrowed from the old witch’s-style brooms, which were placed in the sand as marker points, helping horse and cart tradesmen transport goods on to the island, before the military built the bridge in the Twenties.

If you value your life, do not attempt to tread this path without a specialist guide. “The sea fogs can roll in real quickly, leaving people completely disorientated,” an islander informs me, before slicing a worrying finger motion along the bottom of his neck.

Thankfully, I was moving inland now, to Foulness’s second and most northerly village, Courtsend, past the green hangers of RAF White City, and along a narrow 16th-century road, which is a former sea defence worn down by time.

This is the highest point of the island, just 6ft above sea level, crowned by the aptly-titled Hill House.

Of greater historical interest is the old single-storey pub, the King’s Head, which dates back to the same time as the road.

It served its last pint in 1988 (the old bar is preserved in the Heritage Centre) and its white weatherboards and slanting orange-tiled roof are now fused to an adjoining two-storey abode.

Across the road, a well preserved wooden cottage unearths a military past which easily predates the MoD’s occupation of these lands.

A former signal station, this quaint structure was all geared up to send a distress warning down the Thames Estuary to London by boat, at the first sign of Napoleon’s troops invading from France in the early 19th century.

Before I finished this remarkable journey, I was given one last, sad reminder of how the island’s MoD landlords seemed to be allowing other important empty buildings to rot and die.

The Brick House sits on remote scrubland under a pale blue sky. It is the first brick home built on the island, in the 18th century, with chimney breasts dating back even further, to 1650.

The square walls are naked, chipped and falling apart, while large areas of the roof are exposed to elemental batterings during the long, harsh winters here. And this is a listed building!

“They nearly bulldozed this house to the ground a few years ago,” I’m told.

“If it was anywhere else but Foulness, it would have been revamped, flogged off and have a Bentley and Mercedes parked on the drive.”