All Penalty Charge Notices such as congestion charge fines, parking tickets and speeding fines are the responsibility of the driver.
The Driver is liable for a £10 fine per PCN accumulated during the hire period.
If, as an Owner, you receive a PCN through the post, all you need to do is see the section on the letter that advises what to do if your car was on hire. The usual advice is to tick a box confirming the car was on hire, include a copy of the Hiya Agreement as evidence and to show the Drivers details and then post back.
The fine will then be transferred to the Driver.