‘Hard to say, Jack, hard to say: two hundred and fifty nautical miles assumes we travel in a straight line, but it doesn’t tend to work like that. The sea conditions could knock us off course, and then we have the German navy to worry about…’ He paused, bouncing his pencil on the chart. ‘Today’s what – Tuesday – and my estimate is a minimum of thirty-five hours once we leave Grimsby, but according to these instructions the rendezvous should be between midnight Thursday and two in the morning Friday. I don’t want to get into the area too early, it could arouse suspicion.’

‘What do you reckon then?’

Sid Oliver spoke. ‘High tide’s at about eight fifteen tonight. I think we should delay our departure to midnight. Rather than going in a straight line through Humber and German Bight – they’re shipping areas, Tom, here and here – I think we should head north up through Humber into Dogger and from there down to German Bight. Bert’s right, we don’t want to arrive too early.’

‘I hope you don’t mind me asking, but…’ Prince hesitated, aware that he sounded awkward, ‘aren’t we sitting ducks for the Germans – U-boats and all that?’

‘You’re right to mention that. The danger is from U-boats heading out to the Arctic or returning from it. Their two main bases are here, Trondheim, in Norway and here, Kiel on the Baltic. Fortunately, their routes should keep them north of us. The U-boats based in Hamburg – here – tend not to bother with a lone trawler these days: they worry about exposing themselves, giving away their position, and it’s not worth it for an old rust-bucket like us. It’s not unknown for trawlers to be used as decoys to tempt U-boats to the surface and then attack them. But that’s no guarantee of our safety.’

They set sail a few minutes before midnight, the Northern Hawk slipping silently and unseen through the dock gates into the Humber Estuary, the world around them pitch black, with only an occasional distant flickering light to remind them they were not alone in it.

Shaw and Prince shared a tiny cabin next to the skipper’s, just below the wheelhouse. They remained there the rest of that night and throughout the following day, when the Northern Hawk moved into Dogger and cast its nets. ‘It’s important they keep up the pretence of being a working trawler,’ Shaw explained. ‘And if we leave the cabin, we’ll just get in the way.’

When night fell on Wednesday, they ate in the trawler’s galley and then Bert Trent asked Prince if he’d like to join him in the wheelhouse.