After several hours of trying various things I've come up with this solution to get Last.fm on Pure Flow Radios (tested on Pure One Flow). I couldn't actually find any other solutions on the internet so this may be a first!
1. Download the Last.fm scrobbler client software to run on your desktop and start a radio station playing
2. Download Nicecast for Mac OS-X (http://www.rogueamoeba.com/nicecast/
). This basically captures sound coming out of your soundcard using Soundflower, then broadcasts it using Icecast. Similar solutions should be available for Windows 7.
3. Download and install the latest version of TwonkyServer (twonky.com) to replace FlowServer.
4. Save the local sever address from Nicecast (or equivalent) as a .m3u playlist file "My Radio".
5. Share the folder containing the .m3u file by configuring TwonkyServer.
6. Connect to your TwonkyServer using "Media player" on your One Flow or Evoke Flow
7. Find "My Radio" and play it!
Notes: Obviously there is a slight quality loss as you are capturing the last.fm stream off your soundcard and rebroadcasting it. But if you broadcast as 320 Kbps/Stereo it will be small. This should work for Spotify too. If anyone has a better way of capturing the Last.fm stream (directly?) then please post.