Several hundred British and Afghan soldiers were carrying out an operation in December when they were engaged in a gun battle with 15 to 20 insurgents.
"The guy was wearing a vest. He was identified by the sniper moving down a tree line and coming up over a ditch," said Lt Col Slack. "He had a shawl on. It rose up and the sniper saw he had a machine gun.
"They were in contact and he was moving to a firing position. The sniper engaged him and the guy exploded. There was a pause on the radio and the sniper said, 'I think I've just shot a suicide bomber'. The rest of them were killed in the blast."
It is understood the lance corporal was using an L115A3 gun, the Army's most powerful sniper weapon.
The Armed Forces are gradually decreasing their presence in Helmand, handing over security of the country to the Afghan armed forces. Last month, three major bases were closed or handed over to Afghan control. At the height of the campaign, there were 137 bases across Helmand - now there is only one base outside Camp Bastion, Sterga 2, which is staffed by a company from 4 Scots and the 9/12 Royal Lancers.
The sniper incident was one of a dwindling number of gun battles between British forces and the insurgents. In total, 448 UK soldiers have died since 2001, but far fewer have been wounded in the most recent tour, with Afghan forces now leading 97 per cent of the security operations across the country.
Yesterday at Sterga 2, soldiers said they were looking forward to returning home and hoped their work would help the Afghans achieve stability.
Sterga 2 stands on a plateau above the Helmand river, about 29 kilometres south-east of Camp Bastion. Between Bastion and Sterga 2 is the "protected zone", next to the river, where the local population is living under the protection of the Afghan armed forces.
The camp has only come under attack once, and that was when it was being built last August. "In my tour in 2007, I had seven guys injured while they were actually inside the base," said Lt Col Slack. "We had rocket attacks every day. This base hasn't been attacked since it was built. It feels like it is time to go."
Capt Ed Challis, who is in charge of Sterga 2, said he was hopeful about the future of Afghanistan.
The country has its first round of presidential elections this Saturday, with an increase in violence expected as voters go to the polls.
"I am an optimist," said Capt Challis. "There are lots of things that have changed for the better. You would be a fool to think you can change a hundred years of culture fast, but have things improved? Yes. I believe they are able to take it forward."
He added: "I'd imagine once I get back it's something I'll look back on and sort of realise the historical importance of it - but at the moment we're just focusing on our primary role here."
Highlander Paul Carr, 27, from Paisley, was on sentry duty in the watchtower above the river. He said he was enjoying the hot weather, after the camp was hit by snow in February. "When this base closes, we will go home," he said. "I get a holiday feeling when I think about it."
Highlander Carr was monitoring a small compound on the bank of the river. Camels and goats wandered around outside the farm, with small fields of onions growing in the sun. Poppies were also starting to flower, despite years of programmes to eradicate the poppy crops in Afghanistan.
Abandoned fortifications - Russian installations from the Eighties and older - dot the horizon.
Inside the camp, a company of servicemen and women were working to gather intelligence about the surrounding area.
The information is passed on to the Afghan security forces and intelligence from Sterga 2 aided the sniper attack in December.
Cameras mounted on balloons monitor the fields and compounds for several kilometres around, feeding into an operations room and providing protection for Bastion. The Taliban thought that the large balloon was a "white whale in the sky" when it was first launched.
Lt Col Slack lost one soldier, L/Cpl James Brynin, 22, of the Intelligence Corps, who was shot dead on patrol last October.
Lt Col Slack said he had watched Afghanistan evolve dramatically over the years.
"The price has been heavy for the Army and in particular it has been heavy for the families of those nearly 450 [dead soldiers], and no one is under any illusion about that," he said.
"I will finish my tour knowing one of our NCOs will not be coming home and that is a heavy price to pay.
"Has it been worth it? At my level when I look at security that is here and the way the ANSF [Afghan National Security Forces] have developed, I certainly think it's been worth it."
