“Do you think we are winning?”

The Welsh Guardsman was on his stomach with his light machinegun outside Checkpoint 11 when he asked this of the person lying next to him. He listened intently to the open-ended answer and quietly went back to scanning the canal bank for an expected Taleban attack.

A week ago it was just another irrigation channel in a dusty corner of Helmand Province. But seven days of relentless fighting for British troops has turned the Shamalan Canal into a name that is becoming grimly familiar.

Several British soldiers have died on the canal’s banks in five days, defending a string of checkpoints thrust about 10 miles straight into the heart of a Taleban-held area northwest of the provincial capital Lashkar Gah. A larger, undisclosed number have been injured. Among the dead is the commander of the Welsh Guards, Lieutenant Colonel Rupert Thorneloe, the most senior British army officer to be killed since the Falklands.

The tempo rarely lets up. Attacks at dawn and dusk are routine. One Welsh Guards company was attacked 15 times in a day.

As night falls there is something reminiscent of the Western Front about the battlefield. From their positions the soldiers watch parachute flares cast shifting shadows until somewhere the silence is broken: the first thump of a Taleban rocket-propelled grenade, the rattle of Kalashnikov fire and then the heavier chatter of British heavy machineguns and SA80s. Red tracer bullets make graceful arcs and occasionally there is the heavier crump of artillery rounds and clattering cannon fire from Apache helicopters.

But why is the fighting so heavy? And is it worth the cost?

The position that the British commanders have chosen to push into is an exposed one. There is a single road running along the canal which is the only line of communication and supply. The Taleban know it and are hitting the road from both sides with as many roadside bombs and ambushes as they can put in place.

For the British armoured vehicle crews who are supplying the forward troops, most drawn from Egypt Squadron, 2nd Royal Tank Regiment, it is particularly gruelling work. With mine detection specialists from a Royal Engineer unit which calls itself “Team Rainbow” they must move sometimes at a snail’s pace, sweeping the ground ahead of them for hidden devices.

Such operations, which often come under fire, are exhausting, frustrating and take place every day. To avoid the bombs the British troops have to be efficient, and to some degree lucky, all the time; Taleban bombmakers need to be lucky once.

The Welsh Guards Reconnaissance Platoon has lost almost two thirds of its strength to casualties over the past two months. Many Welsh Guardsmen have fought more firefights than they can remember. Though morale appears solid, some are clearly feeling great strain.

“I just feel cold inside,” one very young soldier said in a quiet moment last week, admitting that the death of his platoon commander some weeks earlier had left his unit grief-stricken. In some firefights, he said with odd detachment, he was finding it increasingly hard not to freeze up, to stay in control of himself.

“Do you think we are winning?” It was a question that was repeated several times by different soldiers, usually away from the hearing of other soldiers. They seemed very certain about the moral case for the war — “we are here to give people a better life,” said Guardsman Craig Jones, 22 — but far from sure of the eventual outcome.

The death of the Welsh Guards commander, Colonel Thornloe, was a particularly awful experience for the unit, which has now lost five men, including a platoon commander and a company commander as well as the commanding officer.

The Welsh Guards is not the first unit to take a battering in Helmand. The Anglians, The Grenadier Guards, 2 Para, 3 Para and 42 Commando are all units to have suffered notably heavily but the concentration of deaths in the Welsh Guards is particularly harsh.

But set against the risks are tactical gains that the British commanders must presumably consider worth the growing cost.

British officers say the canal gives the British forces an effective means of controlling movement into the highly populated areas around Lashkar Gah, the so-called “Green Zone” of irrigated land. The checkpoints act as a block on Taleban movement of men and supplies into those areas ahead of the Afghan presidential elections on August 20. Making sure the elections can be held safely is the primary short-term objective of Nato forces in Helmand.

The operation is a part of a larger and ongoing series of British operations, named “Panchai Palang” (Panther’s Claw), designed to methodically take and then hold the territory around Lashkar Gah. Meanwhile, the much larger US operation launched last Thursday, Operation Khanjar, is pushing 4,000 US Marines supported by more than 180 aircraft into the Taleban’s lines of supply further south down to the Pakistan border.

The Taleban are likely to play for time. In a message to his fighters intercepted by Afghan intelligence in Helmand last week the commander of Taleban fighters in the province, Mullah Naim Barach, told his followers: “If the pressure becomes too much just store the weapons where you can and collect them again later.” The Taleban fight not to win, but to outlast.

Interviewed six days before his death, Colonel Thorneloe said: “It is a big hit to lose someone but there is an absolute understanding that the mission is vital and that those we’ve lost wouldn’t have wanted to be elsewhere and they would want us to see it through.”

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