I know open source map data can be imported into houdini to produce ‘cities’ but try as I might I haven't figured it out, I don't want to buy the tutorial from cmivfx particularly - just to find it doesn't do what I want - si if anyone knows how to convert open source map files (.osm) into something houdini can understand I'd be eternally grateful.

There is an overview of the process at the description of the Houdini XML Based Procedural Cities video (paragraphs 1 to 3 specifically)
http://cmivfx.com/tutorials/view/233/Houdini+XML+Based+Procedural+Cities [cmivfx.com]