Little did their neighbors know
in this quiet corner
—this quiet pretty little corner in the Kent countryside—
at the cottage just over my shoulder
is where they had been growing cannabis
You can understand at the cottage just over my shoulder as a locative phrase paralleling in this quiet corner and —this quiet pretty little corner in the Kent countryside— as standing in apposition to this quiet corner.
It may seem a little odd to have a prepositional phrase stand as the subject of is, but it is quite frequent in colloquial speech when the predicate complement is a phrase headed by where:
At the barbershop down on Main Street is where I got this lightning bolt shaved on my head.
In this corner is where the disobedient child is told to sit. We call it the "time out corner".
... in this quiet corner ... at the cottage over my shoulder is where ...
P.S. The same can occur with temporal phrases:
On the fourth Thursday of November is when we celebrate Thanksgiving in the US.
Locative and temporal phrases can be chained together in order to "zero in" on the specific place or time.
In Philadelphia, in the old section of the city down near the river, on Chestnut Street between 5th and 6th, stands Independence Hall.