Thanks all for the advice.
I did get my brother to stop by Hertz and I also spoke with Rentalcars.com who I booked the car through.
It turned out that as I had prepaid for the car when I made the booking (I'm UK based), that by prepaying the car was in effect held for 24hours - this wasn't stated anywhere on the paperwork, but was what the Rentalcars.com Rep told me over the phone.
Had no issues at Hertz when I went and collected the car last night.