Trying to fix the highly descriptive 'page build error' on an unknown page :-)

That wasn't it, put the liquid tags back into nav.html
Having now install Jekyll on a development server, I now know the issue is with the title conditional in head.html. That has now been fixed.
I have also added _site to .gitignore and changes, params.json, Gemfile & Gemfile.lock to the Jekyll excludes list