Today the builders put the soil stack in from the top floor bathroom down to the basement. Here you can see how the suspended ceilings work to allow pipe runs through the building:

The studwork for the suspended ceilings (and walls for the new bathroom on the top floor) is now pretty much all in place.

I've identified a firm that make replica cast-iron RWG, and they can supply some matching pipe and fittings for the original rectangular soil stack at the front of the property, to replace that dubious leadwork in the front bathroom.

The builders also knocked through into the previously inaccessible area within the basement today, so now we have an extra room!