In the thread about Git Authentication a solution was proposed using git credentials and the fact that the dispatcher is running as root.
See: https://lists.lavasoftware.org/pipermail/lava-users/2018-December/001455.html
I've worked out that even though the dispatcher is running as root, the environment is purged based upon env.yaml that is sent over from the master.
I found that I had to add HOME=/root into env.yaml on the master for the git clone to pick up the files in the /root folder.
Hope this helps anyone else trying to do this.
Pete