La France (SS France) was a lavish French ocean liner launched in 1910. When the First World War broke out, the ship was requisitioned by the French navy. She was turned into a hospital ship in 1916, with place for 2,500 injured troops, and was placed in the Dardanelles, not far from where Mostyn saw her.Usual work, greatcoats handed in, shorts issued, saw big dirigible over Salonica & huge French hospital ship (La France) in harbour, rumours of a move.
Read more about the ship, and see more pictures, in the Wikipedia SS France article.