I am pretty sure I know the answer, but better ask and seem stupid than do something wrong and be stupid 
If I split a way, the two resulting ways remain joined? In other words, I am un-merging, not un-joining? I want to split a way at the intersection of several ways (click here) to make the section of the yellow way east of the junction one way.
As always, thanks for the help!
Tell us first which editor you are using, then we can tell you in detail what you can do and what is the difference between the splitting function and the separate function.
I will be using Potlatch 2 for this.
Using the JOSM editor you have a feature to “unglue” two ways … this seperates the two ways by duplicating the node these two ways are sharing.
As far as i have seen there is no such ftool in Potlatch2, or am I wrong?
Is this the feature you want? Then try JOSM and have a look at the instructions in the OSM wiki to explore its extensive features …
When you split two ways in either Potlatch (‘x’ or cut tool) or JOSM ('p) the two child ways remain joined at the node where they were cut. In Potlatch this will be clear by the outer square box round the node.
To remove the join (i.e., create two nodes instead of one) you can use Shift-J in Potlatch or “g” in JOSM. “j” works in both tools to re-glue the nodes together.
Thanks, that is what I thought, but wanted reassurance before making changes to the most heavily trafficked square in Plymouth!