Just done a bit more fiddling and it seems that my problems were caused by the sessions being before the course start date.
I now have it creating sessions and taking attendance even for days with multiple sessions (I guess this will cause problems if the automatic attendance feature is used but it won't be). I now just want to add the time to the list of sessions displayed- it is something to do with the following line:
<td><?php echo strftime(get_string('strftimedmyw', 'block_attendance'), $sessdata->sessdate); //userdate($sessdata->sessdate,'%d.%m.%y (%a)', 99, false); ?></td>
from manage.php What is the syntax to add hh:dd after the date?