We did the same in August. Very busy site so I did prebook it. Loved having the water very close by too. Not many Uk vans around tho weirdly.Stayed at Mont St Michel last night and again tonight.
Site is easy to find, swipe card access, pitches are ok size, all have hook up.
Its almost full, think 170 out of 180 last night, but is quiet.
17€ inc hook up and water facilities.
Hard not to like tbh.
Cycle track to MSM has gate from site.