After much back’n’forth with FastHosts and the issues with the various levels of PHP support within GRAV I’ve opted to go back to WordPress as my CMS. This will take a while to re-build all the old entries but I was never a prolific blogger anyway.
Update: It actually took only about three hours to build the site and re-create all the blog posts from the GRAV backup. Whilst there is a WP to Grav tool there is no Grav to WP. It all had to be done manually.