Thanks,
so do I have to go through all 1000+ members individually and work out who currently has access and who doesn't and make corrections where necessary? I thought there used to be an interface where you could go into the forum in question and simply select users that were allowed to start a topic within that forum...

Plus, when I went into "Member Management" and selected the member who somehow managed to start a topic in the forum that used to be restricted to just me, it turned out that under "Forum Permissions" his checkbox wasn't ticked! So how could he possibly have been able to post? (He is a moderator, by the way).
The restricted forum is this one:
http://www.superiorpics.com/cgi-bin/ultimate/ultimatebb.cgi/forum/61.html?