yes Maven Home is set on the build engine and i just refer to this via a property in the properties section of my build definition. The in the "command line" tab in the Environment variables section i check the "Add build properties to native environment" however this raises issues with MVN and sometimes JAVA_HOME although both are set on the build engine. (I don't refer to JAVA_HOME in my properties in the Build definition) 

You can dump the properties of your build into a text file to make sure that all values are expanded, for example the external Maven_Home. External environment variables are not available to me in the build definition for expansion using ${env_var_name};

You can, on the build machine, add your folder with the Maven binaries to the path also, which would remove the issue with not finding it.

Other than that, I am not sure I can help further with the given information.

Thanks for your help Ralph. in the end it was necessary to temporarily set MAVEN_HOME via my build script. This solved the problem