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The sound of the roadside bomb going off just behind us is faint, muffled by the heavy armour that protects us as we travel inside a Mastiff vehicle with a platoon from C Company, 2 Mercian.

It could almost have been a car back-firing, but for the sand and debris that swirl through the air as we look out of the tiny blast-proof windows at the back of the vehicle.

The young platoon commander, Lt Jimmy Clark, 26, is on the radio immediately, checking if any backup is needed.

He is told that trained medics are already at the scene, and that we should drive away from any potential ambush or secondary devices.

Later, we discover that the Taliban bomb, laid on a track next to Route 611 in the Upper Gereshk Valley that links the troubled town of Sangin to Gereshk, has claimed the lives of at least 18 Afghans. Five were children, and seven women.

The white minibus taxi that we had seen swerving far off the tarmacked road and onto a local track, perhaps to avoid our vehicle and those of the bomb disposal squad working on another suspected bomb, had driven straight over an improvised explosive device and set it off.

It was one of countless bombs laid indiscriminately by insurgents in the area to attack Nato forces, but just as likely to kill or maim Afghans driving or walking along the sandy tracks by the main road.

British troops nearby had responded immediately, taking the survivors to medical care at the nearest base and then on to Camp Bastion field hospital.

The young soldiers also helped the Afghan police to clear the bodies in the bloody aftermath, a task so upsetting that some simply sat with their head in their hands afterwards, looking into the distance.

The next day, those involved still looked haunted by the carnage.

Many from the 2 Mercian Battlegroup have already seen comrades badly wounded since they deployed here in October, some by insurgent gunshots and others by roadside bombs. But the sight of so many dead, including women and children, was different.

"At the time, I was engrossed in the incident, trying to make sure those who were alive stayed alive, and that the guys dealing with them didn't get injured by further devices in the area," says Captain Jim Fidel, a bomb disposal expert from the counter-IED taskforce, who helped in the immediate aftermath.

"I suppose it's only afterwards when you pause and in the quiet time that you think about it. It's the most horrible thing I've seen in my life."

This area of Helmand has a deceptive beauty. The Upper Gereshk Valley is a landscape of undulating sand dunes, dotted with thickly-walled compounds, with the mountains shimmering in the distance.

It is dominated by the thin sliver of tarmacked road snaking through it, which is heavily used by military convoys and Afghan civilians alike.

Route 611 has become the focal point of the battle between British troops and the insurgency here.

The road was tarmacked by contractors working for the US military, whose Marine Corps took over from British forces in the area two years ago, before they handed back responsibility for protecting the road to British troops.

Although the British counter-IED team had found and defused or destroyed many devices laid by the insurgent bomb-makers, the bomb that blew up the minibus last week had lain undetected, some 20m off the road itself.

At a briefing afterwards, the commanding officer, Lt Colonel Colin Marks, sought to reassure his forces that they had done everything they could.

"Let's get back out there tomorrow. Don't be deterred by this attack. Everything was done that could have been done, and let's not forget that it was the insurgent who laid this bomb. Hopefully, we can prevent similar incidents in the future."

The Upper Gereshk Valley is an area that highlights some of the difficulties that will be faced by Nato as the drawdown of troops begins in Afghanistan, with America due to withdraw 30,000 of its surge troops this year and next.

The British contingent now in charge of protecting the highway is a smaller force than the US Marines before it.

Of necessity, the task given to the men and women of the 2 Mercian Battlegroup, known as Combined Force Burma, is a narrower one: to protect the road, rather than hunt down the insurgents in the hills and the nearby green zone, although US forces to the east and west of their area of operations continue to do so.

Military commanders accept that there are risks - which will have to be carefully managed - as Nato's combat troops thin out and Afghan forces are ultimately given the lead on securing areas such as this.

While some parts of Helmand, such as central Nad Ali, have seen steady reductions in the level of violence, others - including the valley and areas of Nahr-e-Saraj - have not. Some fear that thinning out Western troop levels there too rapidly over the next three years could endanger the security gains made in some areas during the Nato surge.

The thinning out of US troops in Helmand will have a knock-on effect for British troops now concentrated in central areas of the province, with the British government insisting that UK forces will not return to towns such as Sangin or Musa Qala when American troops go.

This week, many of those issues will be addressed when the National Security Council in London considers plans for the rate of the British drawdown in Helmand up to and beyond 2014, when British combat troops are due to end their mission.

Prime Minister David Cameron has said that 500 British troops will leave Afghanistan by the end of 2012, although there has been speculation that the rate could be accelerated.

The UK currently has some 9,500 forces in Afghanistan.

However, with America still providing the bulk of combat forces, the main question remains how quickly the White House will decide to draw down its remaining troop presence. Barack Obama is due to make a further announcement next year, by the time the next international conference on Afghanistan is held in Chicago in May.

Nato commanders have urged caution in the rate of withdrawal, in the hope that a significant international force will remain in Afghanistan for at least the next summer fighting season while Afghan forces are trained, and Afghanistan, its neighbours and the wider international community continue to seek a lasting political resolution to the insurgency.