They headed north out of Lincoln, towards Market Rasen. It was a road Prince knew well. An hour or so later, they arrived in Grimsby, driving through the town and onto the docks. A policeman on a motorbike was waiting for them at a security gate and they followed him as he drove past a large wooden sign for Fish Docks No. 3 barely covered by a frayed sheet of tarpaulin. The enormous Dock Tower loomed in the distance as they headed to the furthest part of the docks, close to the sea wall. They drove slowly along North Quay, the sea wind buffeting the car, until the policeman stopped, his arm outstretched, pointing to a small building.

They climbed to the top floor, to an office warmed by a large oil fire and with views of the dock to one side and the Humber Estuary to the other. Their driver followed them in with their bags and then left.

‘It’s getting dark. We’ll need to close the blackout and put the lights on,’ said Shaw. ‘Before we do, though, come over here.’

He was standing by the window overlooking the dock. Immediately below was a trawler that to Prince looked quite old, possibly not even seaworthy, though he’d be the first to admit he was no judge of such matters. It was painted almost entirely black, with a narrow white band running horizontally around the hull and the name of the ship picked out in white – Northern Hawk, Grimsby – along with its registration, GY512. The same registration was painted on the stern. The deck was blackened with soot and grime, the wheelhouse a dirty white.

‘At the start of the war there were around four hundred and sixty trawlers sailing out of Grimsby,’ said Shaw, who was evidently more relaxed close to the sea. ‘We requisitioned the best ones and they’ve done a splendid job with the convoys. They make excellent minesweepers and rescue ships and the crews are second to none. As the war has gone on, we’ve released more of them back for fishing. The Northern Hawk is ostensibly one of those: it may look like it’s not up to it, but actually that’s deliberate. If it looks like a rust-bucket, the Germans are less likely to be interested in it. In fact, it has a brand new engine and is in tip-top condition; its rather sorry-looking exterior is as much a camouflage as anything else. Ah, there we are – you see those two men coming down the gangplank?’

Prince could make out two figures dressed in black.