The SIMS login has sufficient rights (it's mine - in order to test the setup). I've posted a question back to Capita and their helpful reply was "The task should be set to a user (windows) who has sufficient permissions on the server."
The error message in the event log just says "user has insufficient rights to execute scheduled reports" (Event ID is 0)
I think it's to do with the local security policy and UAC. Although I have the task set to run with domain admin credentials and also set to run with highest privileges it still says user has insufficient permissions. I've started the autoreports.exe in a cmd window with run as administrator but still get the same error message.