25-Aug-2015 -- I checked the tide tables - and it looked like I could squeeze this visit in just before my ferry departure back to England.
For the assistance of future visitors - I walked out onto the sands about 30 minutes before low water. The low tide (at Dunkerque) was predicted to be 2m and the confluence was some 200m from the edge of the sea. On spring tides it will be further from the sea, but on neaps it might never get uncovered.
As I parked up a local hunter mentioned that there might be shooting on the beach (signs warn hunters that they do not have priority and they are to be careful of other visitors - it seems there is a rule that shots must be taken from a seated position). Certainly there was quite a bit of gunfire from a few hundred metres West along the beach. It appears the quarry is not seagulls but some small species of wader.
Even though the it was cold, windy and rainy - typical August - the beach was busy - dog walkers and mussel harvesting.