so if I left the tarmac down - tell me again what you would advise - tiles are 50mm travertine mosaics which will also form base of shower area
My advice is that if you can use a decoupler do so but make sure the supplier is happy with the substrate and the tile format.
If no decoupler is available make sure the primer/sealer you use is sutable for bitumen. I think epoxy is probably the way to go but my experience of bitumen is severely limited so advice from a more qualified expert than me is in order.
Also not sure about the expansion and contraction characteristics with bitumen substrates so this needs to be dealt with. A decoupler would deal with this and I guess a flexi addy would also deal with it.
Just had a thought as well - Instarmac make tile adhesives, primers and bitumen products. Maybe a call to their technical department might help.
Their Ultra proflex SP I think can be used on asphalt.