Ah ha! I've solved both of these now with the help of my wonderful colleague Catherine, Moodle-whisperer! In case anyone else reports the same problems, there were two issues in play here:
1. Why can Dave see this activity in the bar when his group don't have access to it?
The activity was a URL, it was inside a section that Dave did not have access to but was not 'Hidden otherwise' - clicking the little eye to make the specific activity hidden unless he met the criteria solved the problem. *Wonders* does this only happen with URLs?
2. Why does the Student overview show different combinations of activities for different students?
This was unconnected to the problem above: I had forgotten that, in one of the sections, participants don't get access to later units until they've completed the introduction - these additional units were the 'extras' in some students' bars.
Thanks for your time if you're still reading!
Jo
1. Why can Dave see this activity in the bar when his group don't have access to it?
The activity was a URL, it was inside a section that Dave did not have access to but was not 'Hidden otherwise' - clicking the little eye to make the specific activity hidden unless he met the criteria solved the problem. *Wonders* does this only happen with URLs?
2. Why does the Student overview show different combinations of activities for different students?
This was unconnected to the problem above: I had forgotten that, in one of the sections, participants don't get access to later units until they've completed the introduction - these additional units were the 'extras' in some students' bars.
Thanks for your time if you're still reading!
Jo