Peter wrote:
I want to place the soil pipe from the back of a WC pan within the
70mm gap in a stud wall.

Is anyone aware of a manufacturer that makes a suitable pipe which
will be equivalent to a 10cm regular pipe but of an elliptical or
rectangular cross section?

Don't know what configuration you have but can't you 'part-bury' a
regular soil pipe within the stud wall, then build boxing aroud the part
which protrudes (ie 30mm or so?)

David