late late for a very important date
 

I'm in the UK now and have a Sixt rental car due back tomorrow.
It's a prepaid reservation if that makes a difference.

I think it likely that I'm going to be late dropping off the car, I asked a friend of mine who rents much more than I do and he said that it's not possible to extend the rental.

My question is what would I be charged if I'm a couple of hours late? I imagine that it's based on a daily rate, but what rate?

hotels have real-world rates and rack-rates, do car rental firms charge the extra day at a ludicrous rate or the rate you booked the car at.

any advice appreciated

I would call them to extend it rather than just turn up late. That might help with the rate!
When Ihave extended rentals I have been charged the daily rate that I payed

Please let me know if you need any help with this. Gary