Plainly I'm doing something wrong, received very poorly every time I go to the desk even though I try to use the scanner and have all paperwork, passport and cards immediately to hand

Platinum, booked X1 received Tiguan
Platinum, booked X1 received Kia Sportage (which by the way feels like it was made in the eighties and for its size is really small inside !)

for the Kia I was told there were no upgrades available as there were no vehicles free in this 'high season'

Asked if I could pay to upgrade to an X3...

Yes certainly €210 for the week

Going back to Hertz, SIXT Platinum seems utterly pointless especially after being treated so poorly at DUS by a recalcitrant staffer