Sixt complaint: Car Hire Upgrade unilaterally Charged

I had used Sixt car hire for almost 10 years for an annual car hire LHR to LHR. These are normally 2 week periods. Flown from New Zealand, with a chosen vehicle (Skoda estate or similar) and prepaid GBP1200 for the hire inclusive of full cover. On pickup, I was advised the chosen car had changed and I was getting a Jeep Compass. NO MENTION of upgrade. The receptionist advised it was fully insured (as chosen) so no need to worry about contract or damage clauses.

I was just PRESENTED with the signature box for hire. But they wanted a credit card, so I used the debit card I had prepaid with, but at the same time I asked for a three day extension option. No problem as he (the receptionist) advised that a 300 pound deposit for damage was also due, refundable. He clearly advised the charge made would only be the additional, but when I put the card, it kept declining. Unsure of why, I used another card. The receptionist stated that if I changed card, the WHOLE hire would be processed that way and the other card refunded, but I had no choice. DUy took the car and at the first destination, I called up Sixt to arrange the refund and how long that would take.

I had NEVER seen any contract at that point. I hit the roof when the Sixt call handler advised that the charge raised was an ADDITIONAL 1700 pounds. I demanded the hire contact, which was provided by them. I had been charged an additional DIALY charge which made up the additional 1700 pounds – that is almost 3000 pounds for a 12 day hire!!!. They accepted it may be an error, but advised that only way this could be resolved now was at a Depot and to go to the nearest one. Duly did that, Southampton Airport.

They agreed it ridiculous for a 1700 addition to be made to a prepaid 1200 pounds hire, but advised they did not have the authorisation to do anything about it. The only depot that could would be the LHR depot. After a waste of time trying to get through to LHR, AND the fact that I had to be in Milford Haven for a meeting the next day, it left me with no choice but to tell them to take the car back. I then hired with AVIS.

I was promised a call from the LHR manager, but that has not occurred. This needs resolving as I have been forced to cancel a hire service I needed by an absolutely disgusting pack of lies by a hire pickup agent. I cannot upload rental document, but have it.