This is the Phare de La Coubre, the lighthouse that sits on the northern end of the Gironde Estuary in the Charente Maritime in France.
To get here, you have to drive/cycle/walk through the La Coubre forest, which runs along the coast. We arrived at the dunes in the eveing as the sun was setting over the Atlantic. We took a path through the forest which eventually led to a rather steep dune (walking uphill in sand was much harder than I remembered!). Once on the top though, we had a wonderful view!
I have since found out that you can actually climb up to the lighthouse for a small fee, in return you get 300 steps to climb! For opening hours and entry fees visit the lighthouse website.