Thanks Grendel,
It's a tricky shape to make. Your idea with using sweeps also works.
I'm trying a mixture of this with also booling the wheel part - rather than trying to complete it with it there from the start.

About the surfaces...another method! :)

Curves / Extrude
Offset
Extrude
Join
and now you can modify the "yellow" surfaces alone or both to the form wanted