A person is a tatty when they are a know it all and overly self promotes to the point where they lie. If you have a black cat, theirs is blacker. Similar to potatoes these persons can be refered to as 'baked ones' and dependent on how full of it they are they could potentially be fully baked which is the highest form of tatty. Any reference to potato can be used, someone could be a 'jacket of the full baked variety' this would also mean tatty. Someone who hasn't reached full tatty status would be refered to as slowely baking or mpartially baked.
'You know Paul, the one who just keeps talking and talking about how great he is?' 'Yeah I know Paul, he's a fully baked tatty'