When the amount of the rip-off is small, a class action may be the answer. Perhaps you need to contact a good class action lawyer? @:-)

As to the correct GBP being shown, doesn't matter. An incorrect exchange rate has been used to rip you off. No problem to call it a billing error (because it is). As to disputing, just send in a letter saying that a billing error related to the Hertz transaction appears to be on your bill of whatever date and you would like it corrected. If you want to bother, look up correct rate for date at issue and compute the difference. Let Hertz cook up what ever their story is and tell it to the crtedit card company. As I mentioned above, there have been some very big payments over various forex ripoffs when a suit is eventually brought. ^

By the way, after a few letters back and forth, I have often seen the foreign merchant just drop the disputed charge. I know someone who got some Prada glasses in Milan who was taken in a variety of ways. Eventually, she did not have to pay for her glasses. She talked to the store manager later and he told her they did it all the time, she was the first to ever complain. Guess it is usually pretty easy to steal from foreigners.