Searched high and low for a solution to this problem, I realise that this error is the subject of several forum posts but I believe that my issue is a little different.
My issue is user role permission related. If I log on as an administrator I can add new posts as per normal. When I log on as any user with lesser permissions than an admin i.e. editor, I get the "You do not have sufficient permissions to access this page" on the "add new post" page.
I believe the issue could have been caused by the "better WP Security" plugin. I have since deleted this plugin.
This is what I have tried so far:
Ran force-upgrade.php script
Ran 2 SQL queries to check for meta_key prefixes that didn't match table name prefixes. (I changed wp_ to e95_ using "better wp security")
Returned all DB prefixes to wp_
Set permalinks to default
Reinstalled the core
None of these have worked. Anybody resolved a similar issue to this before?