Erel,
Thanks for your rapid replies. I asked the same question on the Scene Builder forum and am still waiting for a reply.
I set virtual memory to the maximum allowable, 4096MB, with no improvement. I then found a reference to the registry and came across:
HKEY_CURRENT_USER\Software\JavaSoft\Prefs\com\oracle\javafx\scenebuilder\/J/V/M/User/Options
The key -/Xmx was set to 999m. I changed it to 512m and Scene Builder runs.
I would have found it earlier if it wasn't for the "/" in the name.
Anyway 5 minutes later and I had Hello World running. Thanks again for all your hard work.
Steve