Er ... sorry Craig, I just noticed that was a cell of type attribute. I do have a rule which calculates an attribute which I display in a cell of type html but the rule is set to "determine automatically"
Thinking about the way html pages get rendered, I would have thought what you are looking for won't work unless you do a page refresh because the html content of a page will be rendered on the server before being sent to the browser.
I assume there are formatting reasons why "C" cant be a straightforward cell of type attriute with the attribute set in the BO as "calculated"
