Just a follow up from before. I rang up Hertz again with this code and saying that I can't get the pre-paid rate with this code so I can't get CDW and SLI included (European rate).

The person I spoke to was very helpful and got the rate that I wanted which included the age differential fee plus CDW and SLI and even beat the rate that I quoted her which I found online which is fantastic, so if anyone has any problems definitely give them a ring.