Sorry Mark, i should have said it was a cover placeholder.
I just remebered i placed the user/group/pages overrides in /templates/JOOMLA_TEMPLATE/html/com_easysocial/covers/user, group and pages but for some reason put the replacement cover in the main ../media/easy_social/default/covers/event folder and swopped the default.jpg name with my own filename for the placeholder, so on update the original deafult.php was obviously put back in place.
I have copied my placeholder file over to the events override folder and all is fine now.
Dozy me, sorry.