Hi Russell,
It looks like you're using different quotes for some fields in the json payload :
[{'C_Some_Field___Most_Recent1': '9372', 'C_EmailAddress': "some'thing@somewhere.com"}]
Single quotes around the first 3 items and then double quotes around the email address. You should be able to use double quotes to enclose everything and then single quotes are allowed when enclosed.
The following diagram describes the state machine on the JSON website (single quotes do not need to be escaped) :
Hope this helps and please let us know if you need more information.
Thanks,
Fred