logicuk - What version of PHP5 specifically are you running?
As for getting this to work, without an error I can't help much, I'd recommend uploading just the php files to your forum (don't install the product), then manually visit yoursite.com/forum/includes/class_activecache.php and class_activecache_methods.php, and see if you get any errors, ofcourse you will need to ensure PHP errors are being shown instead of surpressed.
If no errors occur there (you should see the message "Cannot access directly"), then it's possible another modification is conflicting with vB Optimise, if such is the case I'd be happy to help you figure out where the underline cause is and see what can be done to fix the issue