ssl/non-ssl templates do not have default web pages.
https://www.bytemark.co.uk/docs/symbiosis/website-setup/ says that a new BigV server will have a default web page at http://example.default.bytemark.uk0.bigv.io (for which the documentation says I should substitute my server name).
The server that I created had no such page. Going to https://mainbrain.default.ieiloart.uk0.bigv.io just yielded a 403: forbidden error. In order to check that I hadn't done something to change the default setup, I created a new VM "smallbrain", which gave the same results.
Adding an index.html file, worked as documented in the next step.
What's missing is the Alias directive (snippet below), from both the regular and ssl virtual host sections of /etc/symbiosis/apache.d/ssl.template.erb It's static text, so should be pretty easy to reinstate, BUT, the default page at /usr/share/symbiosis/static/index.html contains at least four broken links into the docs, so the alias should not be fixed before the default page itself is reviewed.
# # This is the directory people are redirected to # if their site is empty. # Alias /bytemark/ "/usr/share/symbiosis/static/" <Directory "/usr/share/symbiosis/static/"> DirectoryIndex index.html index.php AllowOverride All Require all granted </Directory>
NB: I haven't been able to test whether adding a site other than the default site also exhibits this problem.