On Sat, 10 Jun 2006 15:28:05 +0100, Steve wrote:

If the duct is empty (in your case it is) they would tie a string to
the handles of a plastic carrier bag, put the bag in the duct and then use
an air compressor to blow the bag down the duct.

Back when my dad was in the Leccy business, they used a ferret to take a
string into places that the human arm could not reach.