It is a little tricky but who would you claim against the clue is in who stated they would have refunded the differance in the first 2 days - the bank.

Technically Hertz are not in the wrong and any payment is a good will. How to claim from hertz, go back to CS and say you want to log a claim for the amount and stress, inconviniance etc as they did not refund in the correct time scales therefore you lost out. Your also appaled by the CS you want to know why the error occured in the first place. By wanting to know why it happened they wont fob off as easily as if error then you want an apology and a settlement for the for the differance.

Bank - give them a call and say you wish to make a complaint against the bank, they did not reverse the transaction and when hertz refunded you lost out, had bank done its job properly then issue would not have occured. Bank have admited it would have refunded in first 2 days so you can argue this point