This is actually just reporting an issue in the interaction I've noticed between these modules. When I saved a new node, the node was always appearing as Access Denied for anonymous users. I worked out that if I created a new revision, the node was visible. I haven't had time to investigate this further, but I created this uick and dirty workaround, just in case anyone else comes across this issue.

function my_module_main_node_insert($node) {
// create a new revision to get around workbench integration issues
$node->vid = $node->vid + 1;
$node->log = 'Workbench domain initial revision';
$node->revision_moderation = FALSE;
$node->timestamp = $node->timestamp -1;
$node->changed = $node->timestamp;
$node->revision_operation = 0;
$node->revision = FALSE;
$node->created = $node->timestamp;
_node_save_revision($node, $node->uid);
}