Apologies, this is why I love this theme because it is so well documented in-line that I can easily find things! Sorry for posting too soon, because I just found the filter.
Anyone wondering how to do this the function is called ‘x_content_string’.
Then this will work:
remove_filter( 'the_content_more_link', 'x_content_string' );
add_filter( 'the_content_more_link', 'mycustom_the_content_more_link' );
function mycustom_the_content_more_link(){
return 'custom string here';
}