There are no extensions showing within Safari preferences but I have Kaspersky Internet Security installed and I think that adds "extensions" to Safari.
The issue remains if Kaspersky is running or not
The only thing I can link it to is I've started a small business and I've been using a domain providers control panel to edit my website, and also started to use a service called Setmore which I've added bookmarks for. I might try removing these to see if it makes a difference.
[doublepost=1485297219][/doublepost]Just followed some steps I found on another topic which seems to have solved the issue...
With the Finder app active
cmd+shift+g
"~/Library/"
Find Safari and move the folder somewhere else
Reopen Safari