Unfair bussiness practice

I am seriously upset with Thrifty Car Rental Hatfield- i rented a car from them 6 days ago and returned it today- now they claim that there is damage to the underside of the rear back wheelarch- i took a video when i rented the car- it shows exactly what is seen now- if you go on your belly and look underneath the car you can see what seems to be an old dent under the bodywork- now they- Thrifty Hatfield want me to sign for old damage- i am very sure they were aware of the damage - did not write it on the damage sheet- and waited for an unsuspecting Client to blame