I wanted you to know that since you haven't provided us any details for the cPanel or FTP, I just logged in to Joomla administrator of your site and tried to update the sellacious. Let me know if the sellacious installation is working for you after retrieving.
Joomdev are still trying to sort out this issue.
In the meantime, the site has crashed again. It was fine yesterday and I haven't changed anything. Could you take a look at the response from my server and let me know if there is a way to fix it:
'There looks to be a missing class within the code this is not being caused by the server.
cjrrenovations-co-uk.stackstaging.com [Thu Aug 22 18:03:05 2019] [error] [client 18.104.22.168:0] PHP Fatal error: Class 'SellaciousPlugin' not found in /home/sites/7a/6/6a28a854cc/public_html/plugins/system/sellaciousreportscart/sellaciousreportscart.php on line 27'