Sidney Carter certainly did write a song named Dahn Below. Can't remember the whole song but some of it is

Oh it is'nt hard to tell
Dahn below
If it's Bow or Clerkenwell
Dahn below
Oh it is'nt hard to tell
for it's a very different smell.....(lyrics missing)
Dahn below

Or something like that!?

If this is the song you were after I'm sure if you were to contact Mike Sparks(www.mikesparks.co.uk) he would help you with the lyrics. (Mike apparently has contact with the family and copyright holders)

Ob