Thanks Matthias, you set me thinking and I think I've found the problem but not the solution:
Everything is set up correctly for notifications, including the correct email addresses for all the users in the users page of the forum module (they are loaded in from Active Directory). However, even though the users show up in the list, unless the user has actually visited the forum, notifications are not sent to them- even though they appear in the forum user list having been loaded in from the DNN user list.
Therefore, is there some way that I can force whatever is 'activating' the user in the forum without having all users physically log in and visit the forum? Our setup would work perfectly if so, as it can't be guaranteed that they will have visited the forum before a notification will need to be sent to them.
Many thanks,
Tom