Sessions - Not Working

I assume you've copy-pasted the new CI files over the old ones instead of deleting the old system/ directory and putting the new one in its place ... If that's the case - delete system/libraries/Session.php.

Thanks for your reply Narf, I really thought you were on to something then.

I have just checked my system dir for the redundant/stray Session.php but it wasn't there, to double make sure I have just removed the old System dir and added the new CI 3.0.0 System dir in its place.

Any further help would be appreciated.

Well, then there's nothing left but to check your log files, that's what you should've done in the first place.