How do I add an Environment Variable in a VS2013 Setup Installer Project?

Conclusion:
OK, with all the stuff I then went through I ended up with the conclusion that I would be better off getting into WIX sooner rather than later. Since I've also had to get the installer working on a build server and I've got a few other installers that are more complex that I will also have to get working on the build server this seemed to be a reasonable route to take.

Some Links that will help anyone get started with CustomAction in VSInstallerProject
Here are two links I think I'll be using for creating the CustomAction for the installer - I'm going with vbs - of which I know little.

http://www.robvanderwoude.com/vbstech_data_environment.php[^]

https://www.simple-talk.com/dotnet/visual-studio/visual-studio-setup---projects-and-custom-actions/[^]

I also found a codeproject article that may be partly applicable for basic information.
How to Pass Command Line Arguments to MSI Installer Custom Actions[^]

If all goes well I'll try to make a little tip or beginners article.

OK, It's taken me a while to get back to this.
I did venture into WiX and it was the right decision for many reasons.
I then went through a lot of hoops - I think - to create some custom actions and all sorts of stuff to check if the Environment Variable existed already, what the value was and report back if it was an unexpected value - realising that changing it will/may ruin some other programmes. SO - in the end I went with NOT setting the Environment Variable in the Installer but instead setting it in the programme.
This works well - any processes spawned from my program will use these settings by default.

Environment.SetEnvironmentVariable(environmentVariableName, environmentVariableValue, EnvironmentVariableTarget.Process);