Thanks to commenters Adam and Steph for tracking down the thankfully now active Audible.com pre-order page for the US version of Tower Lord: http://www.audible.com/pd/Sci-Fi-Fantasy/Tower-Lord-Audiobook/B00L4IRLG8/ref=a_search_c4_1_3_srTtl?qid=1403229402&sr=1-3. No cover art yet but I assume it’s on its way.

NB. The 73 hour running time must be a typo as Blood Song came in at about 22 hours and whilst Tower Lord is longer, it’s not that much longer.