[Jon]

Hello,
KuFIGr.3./106 operated HE115 aircraft in the area during the summer of 1940 laying mines so they will be a good unit for you to research.
Also a JU88 of 8/KG54 was lost in the area late October 1940 also on a mine laying operation.
It was unusual for an HE115 to operate over the enemy coast, its chances of getting home, if caught by a fighter was slim, let alone offering itself to every land based AA gun as well.
I would think it was either a JU88 or HE111 that dropped the mine.
The term "Mine" is often used and refers in 1940 to the Luftmine in either 500kg or 1000kg. Both were designed for shipping but could be fitted with a parachute for land based targets.
I am sure that the pilot could not chose his target, he either had a sea mine or land mine to drop.
However do not get to hung up on the word "mine" At this stage of the war any big bang was ofetn put down to the dreaded mine when it could equally have been a standard bomb.
Good luck.