I had the same problems Pete, my fix can be found at
www.steadweb.co.uk/joomla/k2-extra-fields-handling-special-characters
The reason why you are missing the double quotes and bracket is because K2 uses JSON (a form of regular data handling) and to split each field up, it uses double quotes, so in your field, the JSON parser thinks the end of the field is after the 10, when really it should be handled to change each HTML entity into its raw form. That's what my fix does. Let me know if you need any help.