The Second World War 60 years on: Where we drew the line

The Red, Blue and Green Lines were built to stop invading Germans. Now only holiday barges disturb the peace

Perched between the railway and the Kennet and Avon canal on Hungerford Common are two huge pillboxes. Partially hidden by riotous brambles, they house a collection of sodden newspapers, discarded jumpers, numerous first sexual experiences and a slip-on shoe. A cross between a public convenience and a public noticeboard, these bunkers are the common's equivalent to a bus shelter. But during the Second World War they were this country's last line of defence against a German invasion.

Following the British Expeditionary Forces' evacuation from Dunkirk, the country was divided up into defence lines. While Corporal Jones's butchers van covered the coast, further lines were created inland and, as a final barrier, Dad's Army's country cousins patrolled GHQ Stop Lines Red, Green and Blue, which followed rivers and canals. The Kennet and Avon canal, slicing the country from Reading to Bristol, was GHQ Stop Line Blue.

In a matter of weeks, pillboxes and tank traps were built along the canal and manned by the Home Guard. What was once a trade link between the North Sea and the Irish Sea became our last chance of stopping Jerry from walking over the whole country. But who did they think they were kidding? Mr Hitler? The canal was in such a state it was more of a last ditch attempt than a thin blue line. Built in the late 1700s to join the River Kennet Navigation with the River Avon, the Kennet and Avon canal runs for 87 miles, with a 57-mile man-made cut from Newbury to Bath. For much of the course between Reading and Hungerford it slices through neatly tailored lawns. But come out the other side and you discover a calm blue country lane, quiet since the railways and roads took all its traffic away.

Heading west, the Great Western Railway, which has been following the canal since Reading, continues to stalk you. For its first 30 or so years the canal carried tin plate, copper, coal, tea and salt. Then, in 1841, the London to Bristol rail link was completed and the canal's profitability plummeted. Slow decline followed and a decade later the railway bought the canal. But it didn't keep up repairs, trade virtually stopped and the canal fell into disrepair. By the time of the Second World War much of it was derelict.

Of course, as is the nature of stalking, you have no idea the railway is there until a high-speed train jumps out from behind the bushes. From here on in the canal belongs to dog walkers, ducks, houseboats and horses. Emerging from miles of uninterrupted nothing, the pumping station at Crofton, dating back to 1812, is one of the few original features this canal has left. With the canal suffering from a terminal water shortage, the station was built to pump water up from the nearby Wilton Water Reservoir. But so what. Just beyond it is Freewarren Bridge. An unremarkable bridge in itself, a lone pillbox stands guard behind it. But with no contemporary landmarks to find your bearings you lose sight of dates and time. This could be 1940.

There are still a large amount of pillboxes along this stretch of the canal. They were built to withstand shelling and so a few measly years and a couple of puffs of wind mean nothing to them. Designed by the War Office, the pillboxes were built by local contractors. Manned only once - a false alarm, needless to say - they were considered immobile tanks. Apparently, the tactical use of the pillbox relied upon it being hidden from sight until the enemy was within effective range of its weapons. They were disguised as railway wagons, dockside cranes, roundabouts, car park attendants huts, even motor vans. But here, as in many other spots along the canal, they were disguised as pillboxes.

Through Wootton Rivers, GHQ Stop Line Blue keeps a low-profile watch on this green and pleasant land. From Milkhouse Water to Honeystreet it sneaks through one English country village after another.

It's frightening to think that the canal would probably have disappeared by now if it hadn't been for the Kennet and Avon Canal Trust. When the war was over and the powers that be stamped the canal useless, the Trust battled for 28 years to rebuild it. Their museum on the Wharf at Devizes chronicles the work undertaken by the army of volunteers who restored the canal. The Kennet and Avon GHQ Stop Line Blue terminates at Bradford-on-Avon. Here, the canal meanders up to Bath and dawdles along to Bristol. Even the river looks like it can't be bothered to go to the sea. With ice-cream vans and tearooms scattering the banks the canal enjoys life as a retired serviceman. But if we ever again have to rely on it in war, in the words of Private Jock Frazer, we're doomed.