I found the fault last night. It was actually the browser (Google Chrome) causing the problem. I had to close all browser windows, Open a new session clear history, Close and reopen to make CB work again.
What i did work out was this. CB was tripping up in the file - administrator/components/com_comprofiler/view/view.users.php - public function startEmailUsers () - It would exit via "error: function( jqXHR, textStatus, errorThrown ) {"
Interestingly there was no "errorThrown" - I assume there should have been.
A note for the developers: This function has 2 possible failure points which both return the same error "Email failed to send" I had to make these different to know which route the code followed before failing. Line 342 and line 350