I went down and had a word with them when they came back today. The Dutch lorry was there ready and waiting for them to arrive. I asked them a few questions and it seems to be like this.
They live at West Mersea and in the heat of the summer, it is too far to ship the iced landed fish from there so they come into the Wet Dock. This time that had been up the North Sea off Sizewell and catch mostly Dover Sole. It goes straight onto a Dutch chiller lorry as they give the best prices but, evidently, their final destination is usually Portugal.