Good news! After reading the wiki (the 2.6.30.5 kernel definately needs installing too!) on installing alpha6 & the kernel (everything worked something seems to be working. I set the IP address for the stick as 192.168.0.50, and I can successfully ping that address from another system. Pinging google.com also works on the sheeva, so far so good

all seems to be working! Now .... how to setup a simple FTP server.....?
Notes (or things I found out on the way while doing all of this.....)
1 As I had a newer sheevaplug I had to change the pid's as laid out here (
http://www.plugcomputer.org/plugwiki/index.php/SheevaPlug_Installer ) . The page didn't seem to specify which file to change exactly, though I managed to find which one. I found it in ~/sheeva/sheevaplug-installer-alpha-6/scripts-linux/openocd/config/interface/sheevaplug.cfg . (Decompressed on my main linux system, not the sheeva!).
2. Installing a new kernel (
http://www.plugcomputer.org/plugwiki/index.php/Install_Prebuilt_Kernels_From_sheeva.with-linux.com ) I installed a new kernel following those instructions but used 2.6.30.5, rather than 2.6.30.3. I followed all the instructions including the setenv's and everything after. The printenv bootargs didn't work, but everything else did.
3. I used the command
cu -s 115200 -l /dev/ttyUSB1 to connect the sheevaplug from the command line, works with no problems.
4. After all this I could add in the auto/iface commands above

.
ljones