Thanks lunat,
Can i just confirm one point.
When you suggest "try to connect it to the soil pipe at some point", do you mean provide the bath waste with it's own dedicated connection to the stack, someway lower than the existing connection?
I wonder that you might alternatively be...
(my first post - i've referred to solutions to other people's queries/problems for years)
I am changing my upstairs bathroom and routeing the bath waste under the floorboards to the cast iron stack.
The existing route is above the floorboards and connects to the stack through a 45degree...