If you have issues where the Sophos sourcetype is not extracting the source webserver & malware signature from web activity events, add this line to pull those events.
I couldn't find a solution for this problem, so here's mine:
"Access was blocked to \"(?<origin>[^\"]+)\" because of \"(?<threat>[^\"]+)\"."
This'll make use of the already created but null fields, origin & threat.
To better assist future readers, please reformat this into a question and a separate answer then accept the answer. Please also explain where the added line should be placed.