Yeah, i use it too and find it very handy.
The only other thing that i have to add to the above is that there are 4 cars within a 15 min walk of Forest Hill station including 2 in the station car park so if the closest one isn't available another usually is.
It's rare that i can't get one when i need one and i never book more than an hour in advance unless i need one for a long period at the weekend.