At midday it's more than 45C in the Sangin valley - an added burden for a soldier carrying a weapon, ammunition, body armour and three litres of water on his back.

Foot patrols can last four hours or they can be planned to go on for days. However long it lasts, though, the troops from 16 Air Assault Brigade have to remain on high alert for the duration.

They wade through irrigation ditches and cover each other as they cross parched open farmland and peer through the doorways of the mud-walled compounds scattered throughout the green zone - so-called because of the lush irrigation that follows the course of Helmand river.

Whenever a patrol pauses, the soldiers find shade, drop to one knee, and snatch gulps of water that is lukewarm and getting hotter by the minute.

The all-out firefights of previous years have all but ceased. “Shoot-and-scoot” is the new small-arms tactic of the Taliban. A couple of men will fire at a patrol - just a few bursts, including perhaps a rocket-propelled grenade - and then disappear sharpish on a motorbike, joining the hundreds that buzz through the countryside.

Every soldier is aware that the choking dust on the trails can cloak the presence of an IED (improvised explosive device), which can be triggered by radio, by phone or pressure plate. Many patrols carry metal detectors.

In the bazaars, the locals - some old and some of fighting age - stare at the soldiers impassively. Here the threat is from suicide bombers: a smiling face approaching a patrol, a car parked on the road.

Recently the suicide bombers appear to have changed their tactics. It's thought now that they're working with the Taliban units; shadowing the firefights, picking a moment to approach, sometimes holding back to strike another day. In other words their deployment is strategic - unlike the impulsive one-stop dashes of old.

Despite all this the soldiers seem resilient. Tom Bright, the young and confident platoon commander who has just guided us through the teeming Sangin bazaar, has no doubts about his role.

“We've got to get out into the local population, let them see us - especially lately, when there's been a lot of intimidation from the Taliban. The more we get out and let them see us, the better they feel.”

With us today is Captain Dan Davenport, officer-in-charge of what the military calls “influence”. He's joined the patrol to inspect the construction of a school - three walls so far and no roof in a dusty compound.

“Everything would happen a lot quicker if we got more NGOs [non-government organisations] in here, helping out,” he says. “We don't have any NGOs working for us at the moment in Sangin, so it's all military-led; and in some ways, that means it's enthusiastic amateurs trying to do this work.”

Back in the FOB - the forward operations base - there's little respite from the heat. The drinking water's warm and living conditions are spartan. Bigger camps have cooks; but in the smaller patrol bases, soldiers prepare their own food with 10-man ration packs. There are no fresh vegetables, no fresh milk - and bread is a treat.

And yet the atmosphere is lively and focused. Even in the suffocating heat, the soldiers go for evening runs, push weights and joke with each other.

When reports of a fatality come in, the news reaches the entire force up and down the valley within minutes, but there's little pause in the soldiers' momentum and routine.

They patrol because that's their mission. The plan is to “own” as much of the ground around the populated areas as possible in order to allow commerce to re-establish itself. Just as important is the long game of winning over the locals.

This week, as a Royal Irish Regiment unit in Sangin prepares to resupply a remote patrol base, the mood is grim and focused. Each soldier has washed, eaten and checked his weapon without discussion.

Vance Crow, a short, brisk Zimbabwean major, goes through the drills. “As we go through the bazaar, pistols - and prioritise for suicide bombers. On the open road, switch to long weapons, eyes on for RPGs [rocket-propelled grenades] and small-arms fire. If we hit an IED, move everyone to a second vehicle but don't stop. If we take casualties and have to dismount, follow my lead. If I can't do it, then follow the second-in-command.”

If the men are worried they don't show it. This is just one run of many this month. There are no incidents: they arrive at the patrol base, where eight Royal Irish Rangers live with 50 Afghan National Army soldiers. The rangers are their mentors, deployed to develop the ANA's infantry skills.

The base itself is smaller than a cricket pitch, surrounded by barriers with machine-gun towers on each corner. In a fortified dugout, seven lads, aged 19 to 25, jibe each other about home - how their team lost, how Lisburn should be called greater Belfast - while Colour Sergeant Tony Mason lies quietly on his cot in the background. All are stripped to the waist because of the heat.

That night they will lead the 50 Afghans on a night ambush, hunting down the Taliban fighters who are hunting them.

Andrew Wilson will be reporting from Afghanistan throughout a week of themed programmes - For Queen and Country - on Sky News, from Monday June 16 at 6.30pm.

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