Alrighty, I've built a new Windows VM, to start everything from scratch - and it seems to build no problem now (although, took a while first build - but that's probably to be expected). So I guess there may have been some conflicts with trying to build from source where the binary installation exists; or some other conflict on my system.

One thing though, when I went to run, I did get an error:

java.lang.NoClassDefFoundError: com/intellij/rt/execution/application/AppMain
Caused by: java.lang.ClassNotFoundException: com.intellij.rt.execution.application.AppMain
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202) <snip>

I found this thread: https://www.labkey.org/announcements/home/Developer/Forum/thread.view?rowId=569 , with the solution: "You'll need to add another JAR to your -classpath argument. On my machine it's C:/Program Files/JetBrains/IntelliJ IDEA 5.1/lib/idea_rt.jar." Which is exactly what was needed, so, dunno whether its worth adding that to the suggested vm parameters in the documentation?