See below for the file requested.
My mysql database was setup by the script that comes with postfixadmin.
As mentioned in my initial post, if I alter the address field in the alias table, delivery fails completely (so it is sucessfully accessing the mysql db).
I have also used this file with the last line commented and all the others uncommented:
user = myuser
password = mypassword
hosts = 127.0.0.1
dbname = mail
#table = alias
#select_field = goto
#where_field = address
#additional_conditions = and active = '1'
query = SELECT goto FROM alias WHERE address='%s' AND active = '1'