Issue: Upgrade to 4.1.0 from 4.0.19 - blank page when running upgrade

Had a bit of a nightmare yesterday after deciding to go from VB 4.2.4 to 4.2.5. Things seemed to have worked initially but then the errors started piling in. Found a couple of problematic plugins that didn't like PHP 7.1 and applied fixes. Many hours later things still weren't working correctly (including a failed install of VW 4.1.0 so I decided to go back to PHP 5.6 and to revert the database to a previous save and to overwrite all forum directory files from a backup. I tried to upgrade Vaultwiki again but after a few 404s when trying to find /vault/install/index.php it eventually asked for my admin password and moved to the /wiki_cp.php?panel=upgrade page but this is blank currently (in all browsers). I can't open my Vaultwiki admin page either and when I click on a link (inside a post) that used to open the corresponding wiki page I see the "Vaultwiki is currently being upgraded" notice. So, I'm kinda stuck and would appreciate any pointers as to what to try next.

Thanks