Thanks for your help!
I went through each of the databases and found no such table called wp_sitemeta.
Then, I noticed in the file manager someone had made a backup of the wp_config file, so i downloaded both and checked over them.
I found these two lines in the middle of the file
define('DISALLOW_FILE_EDIT',true);
define('DISALLOW_FILE_MODS',true);
When i removed them all was well, and i could log in as normal and install plugins.
Disaster averted!
Thanks again for your assistance in resolving this!