The blast from the roadside bomb was a breaking storm of noise and shock that scrambled the senses and shrouded men and machines in a white pall of choking dust.

Long seconds of uncertainty followed, before torch beams swept the evening gloom to reveal the silhouette of the sixth vehicle in the convoy, an armoured supplier, sagging sideways and half off the track. Its cabin was a shambles of metal. Its machinegun turret and its gunner were missing.

There was no sign of a follow-up ambush, but one might be imminent. On the internal radio of the Viking armoured car, an 11-tonne tracked personnel carrier, the crew swore softly and bitterly. “I wish they’d show themselves so I could f*****g . . .” one voice said, trailing off to anguished silence.

With the fate of the supply vehicle crew unknown, members of a mine-detection team clambered out of the blastproof door at the rear and began sweeping with detectors to reach the buckled vehicle, aware that there could be other bombs near by. There was little to do but wait and sweat in the baking belly of the Viking, its air-conditioning broken.

Here, beside the Shamalan canal, a spidery finger of water cutting north to south through ten miles of central Helmand province, is the unreported front line in Britain’s war against the Taleban. At stake is whether the Welsh Guards in the convoy and supporting troops can help to block the flow of insurgents and weapons, before Afghanistan’s presidential election on August 20.

The first five vehicles in the convoy, including that carrying The Times, had passed over the bomb before it detonated under the sixth, injuring but not killing two of the occupants in a separate incident from the similar one that killed Lieutenant-Colonel Rupert Thorneloe.

“I didn’t hear a sound,” the driver said later, shocked but unhurt. “Everything went black then white because the radiator burst and the cab filled with steam.” He declined to be named for fear of alarming his family.

About 25 minutes after the blast the injured men were on an American Blackhawk rescue helicopter.

For the soldiers who remained in the convoy it would be another 24 hours before they covered the two miles to the next British base. They sat in their vehicles overnight, waiting for a recovery vehicle to tow out the blown-up supplier. By 8am the Taleban had gathered for their first attack. Local people could be seen fleeing.

Their first strike was a rocket-propelled grenade. It smashed into a wall 15ft to the left of the lead Viking, the blast catching the top gunner in the face and breaking his nose. The blast goggles he was wearing saved his sight as he staggered out of the turret, shaken and bewildered.

Skirmishes continued through the day, the Taleban creeping close from all sides to launch rocket-propelled grenades and Kalashnikov fire.

“I’ve three men in the treeline with AKs,” Trooper Dan Clarke shouted from the top turret of a Viking, his words followed by a shattering cacophony of gunfire from the British. As more rocket-propelled grenades came in, a British soldier answered with a shoulder-fired rocket.

Then, as the recovery vehicle, escorted by two Mastiff armoured cars, neared the convoy, the lead Mastiff was caught by another buried bomb. Its heavy armour saved the crew, but it had to be recovered by the vehicle it was escorting.

“They’ve had us today,” grimaced one of the Viking crew — a tacit acknowledgement of the tenacity of the Taleban that is common among British troops.

Sweat-stained and exhausted, the convoy rolled into Checkpoint 7 on Tuesday evening. “That was an epic,” grinned Sergeant Sean Powell, a Royal Engineer. “Another epic,” corrected another soldier.

Others have not been so lucky. Taking the canal and holding it using a series of outposts as part of an operation called Panther's Claw, has proved costly. In the past week the 1st Battalion The Welsh Guards, supported by The Light Dragoons and elements of the 2nd Royal Tank Regiment, seized the banks of the canal and began to build checkpoints at the 13 bridges along its length — helping to cut off the movement of Taleban fighters, weapons and ammunition into the highly populated districts west of Lashkar Gah.

Another aim is to split the Taleban from an ideological position — to the West of the canal are insurgent strongholds deemed so militant that they are irreconcilable, but to the East the hope is that some Taleban supporters may be brought on side.

The mission has been co-ordinated with the huge mobile assault by US forces in the south of Helmand, which began yesterday.

As darkness fell on the banks of the Shamalan, flares drifted into the air hoping to illuminate Taleban on the move, and bursts of tracer fire and artillery explosions lit the darkness as enemy fighters probed the line.

In Checkpoint 11, a mud-walled compound in which Sergeant Matthew Parry, 31, commanded a part of Prince of Wales’s Company, they have become used to a daily routine with attacks at dawn and dusk.

The days are spent chasing the shade, as the men put it, as temperatures soar above 40C. Weighed down with weapons and body armour they need to drink 7 to 15 litres of water a day. Farther north, the second company of the Welsh Guards was attacked 18 times in a day as it occupied Checkpoint 7.

The Welsh Guards Reconnaissance Platoon has suffered most. On Wednesday the unit’s second-in-command, Captain Chris Lamb, was shot in the knee. By Friday, 19 of the 30 men in the platoon had become casualties since their deployment to Helmand two months ago, though none has been killed.

The Welsh Guards have now lost their commanding officer, Colonel Thorneloe, and another senior officer, Major Sean Birchall, in the past ten days.

And that is despite the British possessing an ever more sophisticated arsenal of weaponry. A faint buzzing sound betrays the invisible eyes of British Desert Hawk drones. Often they work in tandem with Apache attack helicopters.

When two men swam the waters to plant a roadside bomb outside Checkpoint 11 on Sunday night they were picked up, first through intelligence reports that such an attack was imminent, and then by the footage from the drones. A winking light in the sky and the jackhammer clatter of an Apache 30mm cannon firing from 1,000 metres were all that marked the last ten seconds of the bombers’ lives.

Against the British is a low-tech but determined and often hidden enemy. The roadside bomb is now the Taleban’s main weapon. Bomb disposal experts credit the Taleban with greater skill than the IRA.

The bombs are often triggered by an improvised buried pressure plate, triggering the explosive when the vehicle passes over it.

The British bomb squads are clearing an average of 400 devices during a four-month tour. It is a particular irony that, according to one disposal expert, the Taleban now make most of their own explosive, producing high-grade munitions from fertiliser often handed out by the British to farmers under schemes designed to woo them away from growing opium. This is denied by the Ministry of Defence.

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