@dobey
Sorry about the delay in replying
After
apt-get install software-properties-common
apt-get update
add-apt-repository ppa:deadsnakes/ppa
apt-get update
apt-get install python3.6
seemed to finish ok ;
but "python3 -V" still gave Python 3.5.2.
Following your reply I looked further and realised that I needed to use the command "python3.6 -m venv homeassistant " to correctly (?) use python 3.6