Funckyfido, if Lonercom's answer satisfies you then feel free to ignore this (and mark the thread "solved"). But if all you want is display, I think you can indeed calculate the difference between your start and end date; just do the subtraction:

=B1-A1

Then change the format of the result to "[h]:mm". The brackets around the 'h' cause it to display all the hours involved, not topping out at 24. And after that, of course, adding them up is just as simple as a SUM function.

As for skipping weekends, I'm not sure what Lonercom is reading but if no work takes place on the weekends then the start and end timestamps will reflect that; no need to do anything special. But maybe I misunderstood that part of the question.