I thought the term 'cousin' was only reserved for children of my direct aunts and uncles ie. my mum or dad's brothers and sisters. My aunts and uncles are on the same branch of the family tree(just like my mum, dad and grandparents).   But my grandparent's brothers and sisters are on a different branch, right?    So are Sara, Mark, David and Emma all my cousins?

Mark, David and Emma are on a different branch, but Sara is conected to you through your grandfather.  Because Mark, David and Emma are connected to Sara, that means they are related to you

Sara is your great aunt.