"I have a proper, grown up professional contract which means that I have no set working hours; I just have to get the job done. "
I work like that as well when I can. It doesn't mean that I'll take calls or respond to email outside of normal hours. I get the most done when I can turn the ringer off and spend the required amount of time concentrating on the work. Every interruption can mean 10 minutes or more of lost time "reacquiring the target". If I can get into a zen flow, I can get a whole bunch of work done at one sitting. When I get to a natural stopping point, I'll come up for air. Sometimes that's 20 minutes, sometimes a couple of hours.