This area is swarming with muggles 24/7. Of course, there are peak hours, as well as off hours, regardless...Muggles will be around all the time. Timing is everything with retreiving this cache. Even the sound of metal on metal scraping as you retrieve this cache will make you groan.
The degree of difficulty is soley due to the location of this cache. Otherwise, it's a super easy find.
Good Luck!