The message says
and it works as follows:
And of course
Credit where due: There were a couple of errors in the first version of this answer, which I have fixed after Rubio kindly pointed them out.
Discredit where due: I originally claimed that one element in the process was "sometimes slightly sketchy", but the sketchiness was in fact all in ...
some clues to get the ball rolling...
I will let your door be spun (5)
I'm a letter, not the start, (5)
I'm a smell that you should ban (4)
I'm with ocean in part one (6)
I'm before you'd say in Rome (4)
Canoeic palindrome, alas, (5)
I'm three less, if upside down (4)
I can swirl, flow, stick and foam (4)
And one ...
If you look at the map, it broadly resembles 2 clocks with hourly positions north and south of the river.
I labelled locations North:1, North:2 etc.. according to their hour on the clock.
I separated the data into north and south, and then through a little trial and error discovered the relationship that that the location is related by the formula:
To submit something that no one has yet suggested, Rb2..Nc7# is valid but not mandatory for black because the option of Bxf7 is always in play unless black's king is placed into check.
The solution hinges on F7 because outside of A1, it is the subject square of which Black has the most options.
A1 tells us two things when combined with F7:
You must check ...