When I set up a score I like to have bars already there. How do i get rid of any extra bars that I do not need when i have finished the work.
Choose Write > Trim Flow.
I take it that I have to highlight the bars first?
No, the Trim Flow command simply removes any empty bars from the end of the flow. You can execute it any time with any selection, or with no selection.
What happens if you’ve got a Layout for one or more parts, which has 8 bars’ rest at the end of the piece, but there are 2 more bars in the document beyond that, which need to be removed? 
Trim Flow trims the flow, (doh…) not the layout!
If a layout doesn’t contain all the instruments in the flow, and ends with some bars of rests, those rests will appear in the layout (which is probably what you want).
Great, just did it with one click!