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To fix things for the moment I simply removed the jdk folders.
And then checked that the reported version of Java was 1.6 on each Mac.
Web applets still use the up to date, secure version 1.7 plugin.
On two of the Macs, as soon as I fired up the Citrix app, the Java vpn window would show 'error'. Then I remembered that I had done some Java development in the past and installed various jdks from Oracle so I ran: That was on the working Mac.
Then I remembered the difference between 'System' Java, Java plugins, and Java development kits.
Simply put, you can have multiple versions of Java in different places.
What was happening on the not-working Macs was that the jdk versions were being used, and the Juniper vpn client won't work with them.
Recently, I was trying to get a Java applet to run in the same way on 2 i Macs and my Mac Book Air.
The applet is a simple vpn client from Juniper that lets me access a Citrix Desktop from any Mac that I can install the Citrix receiver client on so I can work on 'Company stuff' from a large screen i Mac when I'm sat at home or from my Mac Book when I'm on the road (it works fine over 3/4G).
The first thing is that you have to do some configuring of both Java and Safari to get the applet to run at all.
Once that was all done, I could log in from all my Macs, fire up the applet and establish a secure connection. I made sure that the Safari and Java preferences were set the same on each machine but still no joy.