Having mastered the arts of survival in the Arctic – living in snow holes (a man-made ‘cave’ carved out of the snow), skiing, marching on snow shoes for five kilometres (three miles) with 30kg (66lb) of kit on their backs, personal care (such as not touching exposed metal) – it was time to move on to the fighting element of the training.

Cetus reached its climax on the Trondes Peninsula on the northern side of Harstad harbour – and home to a formidable relic of the Third Reich.

Batterie Theo – today known as Trondesnes Fort – was part of the Atlantic Wall, the chain of defences built by the Germans from Biscay to the North Cape to prevent the Allies storming Fortress Europe.

The battery and bunker complex serves the ‘Adolfkanonen’ – Adolf Guns – four mighty 40.6cm (16in) cannons which could hurl a 1,030kg (2,300lb) shell up to 56 kilometres. One of the guns is preserved in working order by the Norwegians to this day.

With the Norwegian Army defending the fort (currently home to their coastal rangers) 45’s Yankee Commando staged a three-hour-long assault on the imposing fortification, launching simultaneous attacks on three enemy positions.

The attack ended with the Adolf Guns in Royal Marines hands before three Norwegian Huey helicopters and Fleet Air Arm ‘Jungly’ Sea Kings of 845 Naval Air Squadron swept up the fjord to extract the Yankee men – and Cetus 13 was completed.

“It’s been good to put into practice the skills we’ve learned over the past month. This is my first time in Norway and fighting in the extreme cold is a totally different challenge entirely,” said Marine Tony Curtis, aged 19 of Newcastle.

45 Commando are the UK’s experts in cold weather warfare. Based at RM Condor in Arbroath, their year-long stint as the on-call commando unit ends in May when the green berets of 42 Commando at Bickleigh, near Plymouth, take over the duty.