The bathroom re-fit is about to move from plan to implement but I still
have a couple of questions:

Can I run the toilet soil pipe between the joists to the SVP? what's the
minimum fall/slope? Is there any type of pipe that would make this easier?

I'm going to sit the shower tray on the floor (bedded on marine ply and
mortar?) and run the waste between the joists too. Is there any reason
not to join-up with the bath waste before the SVP? Is there a shower
waste that can be cleaned from above? Is there any point using one of
these "vortex" shower wastes? What's the minimum slope?

I want to run the basin waste in a new stud wall but it's going to be a
real pain to drill the (big) holes in all the studs, the alternative is
to saw (big) notches once it's built. Is there a narrower pipe that
people use in these circumstances?

--
Dave S