Can anybody point us in the direction of a web page which explain how the expression "Is the Coast clear" and variations came about?
My gut feeling suspects it came from the smugglers in 18th century Cornwall /Devon?

Didn't actually find one, but...

This one gives one meaning as "None of the coast guards are about":
http://phrases.shu.ac.uk/bulletin_board/6/messages/792.html

This page doesn't want to load for me, but google's cached version says, "If you say the "coast is clear" you are really saying it is a safe mapped coastline, safe to sail."
http://ibnd.com/columbia/phrases.html

This one has some books listed at the end that might help:
http://www3.teleplex.net/timonier/speaks/book01.html

This one has some early occurrences of the phrase that don't really carry the same meaning:
http://phrases.shu.ac.uk/bulletin_board/6/messages/792.html

It's from rather earlier than the 18th century. "The king intends to go to Calais but we must first clear the coasts" appeared in print in 1530. It is obviously a reference to the need to ensure the landing-place was free from obstruction, not least by enemies. In 1584, the phrase "seeing the coast clear..." first appeared as such.
The OED defines 'the coast is clear' as follows: "ie of enemies who would dispute an attempt to land or embark". The later smugglers would simply have borrowed the already-existing phrase to refer to the absence of excisemen.