I just booked a car in London through Hertz for pick up at my hotel - Imperial Hotel Russell Square. Hertz has an office next door. I booked the car with Hertz through www.carhire.com.au (www.vroomvroomvroom.co.uk), who I have used on many occasions without any problems. The cost for one day is approximately 20 pounds. I went to the Hertz website to query car hire on the same date, and the costs for one day for the same class of vehicle were up around 500 pounds - why the difference?
PS. I am aware of other transport options, but in this particular instance they would not be practical.