Hi Graham
Travertine outside is lovely stuff but uk weather is no good for it and it freezes and snows in Spain. As Trav is porous it fills up with water and can crack with frost. You are right there is a great deal of cheap Trav and most of this is low grade (softer with more voids) low quality .
Personaly I would put nothing thinner than 15/18mm Trav outside correctly laid as it would be asking for trouble.
tiler