Welcome to the forum Karl.

I've just moved your post to Car Hire where it's more likely that our resident car rental guru Roger will spot it and hopefully be able to answer your questions.

As to what time to book the car, it usually takes around an hour or more to get from the plane to the car rental desks so that should give you roughly your pick up time. Normally if you're a bit early or later, unless it's really peak time and you're early, then there's no problem.

When bringing it back, the car rental goes in 24 hour blocks - so if you pick up at 6 pm then as long as you bring the car back before 6 pm you'll be fine.