Shortround6
Major General
It's to be expected the port will need to be reseeded with mines on a weekly basis. As long as the seaport remains closed Germany wins.
try nightly, or at best 3 times a week. The British only have to clear a path 500-1000ft wide. The Germans have to "reseed" with in that width or additional mines are useless. Just to make it interesting the British can also lay out a dogleg approach to the port given enough depth of water so there isn't a 2-3 mile long straightaway to drop the mines into.
Sweeping techniques varied but even the magnetic mine was forseen and provided for including aircraft.
Vickers Wellington DWI
This is not a difficult conversion and the more the Germans rely on mines instead of normal bombs the more effort goes into mine sweeping instead of other weapons.