If you want to find more information on the ship, the World Ship Society publication "Short Sea: Long War" by John de S Winser has a section on the Isle of Guernsey. There are several photos in the book, but they are all of her later service as a Landing Ship Infantry (Hand hoisting). However, photographs of her sister ship Isle of Jersey also appear in her gueise as a hospital carrier, and I found a number of pictures Isle of Jersey in the Imperial War Museum photo archive.
I made a model of Isle of Guernsey as a hospital carrier at Dunkirk for the Finewaterline display at the 2010 Telford show and subsequently a resin kit. Pictures are on the Finewaterline website in the Gallery section on dioramas at
http://www.finewaterline.com/pages/gall ... /dmIOG.htm.
Hope this helps - these ships did a great job often under attack despite their Red Cross markings, so you have a grandfather to be very proud of.
Best wishes
Don