Ok. I managed to get this working today.
I passed in CustomActionData from my WIX files
<CustomAction Id="SetPathInst" Property="EnactorInstaller" Value="DataKey=[SourceDir];DataKeyInst=[INSTALLDIR]" />
which I could then read in my Vb using
Dim srcPath As String = session.CustomActionData("DataKey")
Dim srcPathInst As String = session.CustomActionData("DataKeyInst")
I had to ensure my CA execute was set to deferred. My above example allowed me to pass in multiple property values within one single custom action. I then also had to set the property of this CA to point to the ID of my main CA and set this to execute first.