... If you already have a "mail" icon on the desktop, right click on it and choose edit.
When the window pops up, enter seamonkey for the program to open, then enter -mail in the argument to pass area.
open /usr/local/bin/defaultemail as text
Use this inside and drag that to the desktop.
exec seamonkey -mail "$@"
I originally opted for the second method as I, at that stage, didn't have a desktop icon. The folly of that is evident in my subsequent posts.
The correct course of action and it only took days for the penny to eventually drop was, in fact, the first method.