You've set your rule in the negative and the condition site value is not escaped.
Try substituting the ! for a ^ and escape the periods (full stops) only in the condition. That is, place a backslash (3) in front of each period for the site only - tumblr.jatex.co.uk
I tried to show you, but this site won't let me use those characters. It sees it as code (which it is) and blocks it.
DO NOT escape the rule.
I'm also assuming, your server supports mod_rewrite.