BMW dealership ********* in used car sales

I Purchased a used vehicle from SMG Century City in September 2016, the car was a different car brand to the one they sell (probably a trade-in). The vehicle was sold to me as having 118KW out put and this was the selling point at the time of purchase and i was satisfied that by purchasing from a BMW brand i would be buying from a reputable franchise which does their due diligence, they assisted me with obtaining an extended warranty etc.. we did the paper work and contract, Tax invoice and my confirmation of cover (insurance) all had the description of the goods i purchased and associated VIN number 118KW. 2 years later i want to sell the car and upon inspection the dealership im trading in my vehicle says sorry sir i have to offer you less for your car because its a 90KW and not the 118KW! to my shock and dismay im told i would have to accept less. i took this up with BMW SMG century city who disputed the fact but all documentation states this clearly. i having been paying a higher insurance premium due to the incorrect classification, and inflated monthly finance installment due to the increased capital and on top of everything I GOT THE WRONG CAR! it has been two months and i have not been able to resolve this issue, i cant trade-in my car to end this nightmare (wrong description on settlement), higher shortfall (no refund) and im totally disappointed by how ********* BMW dealerships are!