Thanks for confirming it should work Erich.
I had an installation problem with the IDE before but just got that working so circling back to this now in case that was the problem.
When I try it now with the Contact checked it fails with this error:
An error occurred while collecting items to be installed
session context was:(profile=DefaultProfile, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
No repository found containing: osgi.bundle,org.eclipse.cdt.testsrunner,8.1.1.201811241055
No repository found containing: osgi.bundle,org.eclipse.cdt.testsrunner.boost,7.0.1.201811241055
No repository found containing: org.eclipse.update.feature,org.eclipse.cdt.testsrunner.feature,9.6.0.201811241055
No repository found containing: osgi.bundle,org.eclipse.cdt.testsrunner.gtest,7.0.1.201811241055
No repository found containing: osgi.bundle,org.eclipse.cdt.testsrunner.qttest,7.0.1.201811241055
No repository found containing: osgi.bundle,org.eclipse.cdt.testsrunner.tap,1.0.1.201811241055
I'm was using the exact same selection as you (Show only latest version, Hide installed, Contact all updates sites...).
Trying again with "--All Available Sites--" selection instead of just "Eclipse 2018-12 update site" but that gives:
Unable to read repository at http://www.pemicro.com/eclipse/updates/content.xml.
Unable to read repository at http://www.pemicro.com/eclipse/updates/content.xml.
Connect to www.pemicro.com:80 timed out
I have an idea...