Hello Leo,
, I have Monchaux-lès-Quend, north of baie de Somme and Monchaux-Soreng, south of baie de Somme, too!
In my own lost list it was just written Monchaux (without "e"). The nearest of Berck being Monchaux-lès-Quend. South of Berck, North of Le Crotoy. I suppose that one then.
Cheers, Franck.