Sixt Car Hire www.sixt.co.uk Review

I booked and prepaid for my vehicle, I even received an email from them requesting my flight number so that the car would be ready and waiting for me, how professional. I arrived at the Gatwick South desk and was immediately offered an upgrade vehicle for a 'special' rate. I declined, I didn't need a bigger vehicle. Now we get to the unprofessional bit, immediately after declining the upgrade vehicle the agent told me that he actually didn't have any other vehicles so it was mine complimentary. Once I had had finished my travels I checked up the rate on the vehicle given and it was nowhere near the rate given by the agent! His 'special' rate was in fact the book rate for the vehicle.
Why did they not just give me this vehicle complementary in the first place? If I had been suckered into paying for it I would be reporting them for miss selling, I did complain but it fell on deaf ears, they offered me a free upgrade on a future hire but it was refused, there will be no future hires!