Dave, are you actually using a node override for that folder which you want to override? Could you post the part of your override.ini.append so we can look at it?
In the meantime, here is an example of overriding a specific node which is of a class that also has an override associated with it:
Once again your advice has been great :) I've sorted it.
I was using the template editor thing that is under 'Set Up' in the Admin, and I couldn't see a way to change the order of the overrides so I didn't think it was important about the order but when you said I should look at the order, I changed my override.ini file. Just curious... why is there no way to change the order in the admin interface?