Having discussed the matter with some of the better working girls in the Manchester area here is something to think of.
A decent girl will not do back to back bookings, she will have at least an hour in between 2 clients. This allows her to accommodate the clients running a bit late, gives her enough time to get showered, wash her hair if she got sweaty during the session, change the bed, put make up on etc. If she is doing it properly she needs an hour in between 2 bookings. So getting ready for the first one then an hour with the first one then getting ready for the 2nd then an hour with the 2nd... That's 4 hours of her time for 2x1 hour booking. If she is a single mum too than it is 4hrs+ babysitter cost too. With a 2hrs booking she will need an hour before to get ready that's 3hrs of her time (and 3hrs babysitting cost in case of she is a single mum).
So say she is 100/hr and 160/2hrs then with 2x1hr booking her hourly rate is 50 (out of that she will have to pay 4hrs babysitting) and a 2hrs booking would come to 53.33/hr (and only 3hrs babysitting). Obviously it's true for even longer bookings but anything longer than 2hrs some girls would find too long, too tiring etc.