“GRENADE!” I ducked, absurdly. After the tiny explosion, the marines attacked the building.

This was the high point of an exercise for the Poole detachment of the Royal Marines Reserves.

They stormed a set of houses at 7am, in sub-zero conditions, after a night sleeping outdoors.

Men from all stages of training were spending the weekend at Hampshire’s Longmoor Camp.

How did all this feel for Marine Robert Leigh from Lower Parkstone in Poole?

He was doing the real thing in Afghanistan with 40 Commando during the summer. The 31-year-old looked tense as he talked about it.

“The hardest thing to deal with was the casualties,” he said.

“You can’t prepare yourself for that. You can’t train for that.”

The Commando unit lost 14 men, the most of any British unit yet deployed, and most men had near misses.

A bullet took a chunk out of Marine Leigh’s machine gun. He said he has given the incident no thought since.

“Some guys were luckier than me, other guys were less lucky. I just put it down to chance.”

Marine Leigh is an account manager for a bank. He joined the reserves to test himself against an elite standard and many Poole men said the same.

The Phase 1 recruits were on the first exercise, learning basic skills like pitching bivouacs.

Peter Collins, 27, from Corfe Mullen, a painter and decorator, said: “It’s all about the Green Beret – am I good enough for it?”

They must train for around 16 weekends and more than 40 evenings, and the whole process takes 15-18 months.

The 25-strong Poole detachment is based at Hamworthy camp, home of the SBS.

They are heavy weapons specialists and some of them spent part of their weekend firing machine guns and practising with Javelin anti-tank missiles.

The atmosphere appeared calm and friendly. The new recruits all said they were enjoying themselves.

One memorable sight was that of a heavily built instructor stripping naked a few feet from the new recruits to show off the wet clothing drill.

The Phase 2 recruits did a nine-mile march on the Friday night, carrying 40-50kg each, and took part in the attack.

Many of the Poole men are highly intelligent. Major Cliff Dare said 40 per cent of Royal Marines are qualified to be officers.

Phase 1 recruit Alistair Caldwell, 18, from Lymington, is going to study international relations at the University of Plymouth.

Chris Luche, a Phase 2 recruit, is doing a masters degree in Forensic Archaeology at Bournemouth University.

And Hugh Wright, a 32-year-old Phase 2 recruit, is a trainee plastic surgeon at Salisbury Hospital.

He worked on wounded soldiers but said that inspired him to join up because they were such motivated patients.

Recent detachment trips included mountain training in America with the US Marine Corps, and next year men will go to Norway and the Falklands.

The men also take part in the beach assault during the Bournemouth air festival and Major Dare has done the commentary for the past two years.

The detachment has sent men to Iraq twice, and Afghanistan four times.

Two men are going next April.

Lance Corporal Martin Whittle from Ringwood, an IT manager for Aldridge Brownlee solicitors in Bournemouth, is already mobilised with 42 Commando.

And Marine ‘JJ’ Breen, a trainee chiropractor at the AECC in Boscombe, is with 45 Commando.

The attack was as realistic as possible, full of gunfire, smoke and explosions, but it could not ever convey the real thing.

Marine Leigh remembered the strong physical and psychological impact of seeing his first IED go off, around 10 metres away, and realising just how powerful they were.

He said: “Far from putting you off, it makes you want to get out there on the ground and stop them doing it.”

He added: “Being frightened isn’t really an excuse. You do what you need to do.

“When you are out there, you only have each other. Someone has got to do it – someone’s brother, someone’s son – and I couldn’t think of any reason why it shouldn’t be me.”