On board the Vandyck was a certain Walter Purdy who became known as the Colditz Traitor http://www.echo-news.co.uk/news/1941694.quiet_man_of_thundersley_was_the_colditz_traitor/One of the reasons your father may have beeen on the road gangs is that there were two POW camps at Marlag Ind Milag Nord , Westertimke (Tarmstedt). they were not actually completed when prisoners were taken there, so the prisoners had to literally build their own prison, one was for merchant seamen, who under the genevre convention, should have been repatriated, but were instead eventually put in the camp built for non combatant civilliams, even though they were under royal navy command. the germans did at one time offer to do a prisoner swap of merchant seamen, but Churchill refused, reason being he thought that if the german ones were released they would be pressed into crewing U Boats and warships,