Ok i've updated the permissions on the files in the cache directory and cleared the cache. The Who's Online island has now changed.
What I don't understand is Gizmo's comment:
"And theres a simple way to tell, look at the Who's Online page, a bot will have a "user agent" string that tells you where it's from, and it'll show in the "guests" or "bots" section (it'll show in the bots section if you tell your ubb which UA's are bots)"
Where's the Who's Online page?
Much thanks everyone.