But I can’t get the curve to follow the complex rise of the staircase, perhaps I need to subdivide the curve to get more control points? But I’m not sure if I’m even on the right lines.
I think you could do it with curves, maybe with some subdividing and manipulation of the vector handles to control the influence, but another way might be to use the skin modifier, and maybe do a little clean up around the corner geometry.
Making the banister out of curves: Create two bezier curves; path along banister and profile of banister. Object center of banister profile will travel along the path curve. The way banister profile travels is along the profile in its objects Z-axis. Construct those accordingly. Keep Z-axis of path up as well.
To create banister, select curve path object data. Call the profile curve under Geometry, Bevel Object. Then set Shape, Twisting to Z-Up:
