Cast Iron Stack Pipe: Moving toilet but no idea how?!?

Hi,
I'm trying to redo my bathroom in my 2nd storey flat (7 storey block) and would really appreciate some help. The toilet needs to be shifted and a bath positioned where the existing toilet waste pipe is, but unfortunately I think the stack pipe is cast iron (just been told it's galvanised steel) and so I am having a few difficulties knowing the best way of doing it. I'm also having difficulties explaining it so I'll attach some pictures.
This is the stack pipe and existing toilet...

and this is a rough plan of how it currently is:

The red is the stackpipe, blue the toilet waste pipe (I think this is also cast iron or galvanised steel), and the green is the toilet.

I would like to change the bathroom so it is something like this...

where the grey is a bath.

Unfortunately I do not know how I can connect the toilet to the stack pipe in such a way that I can get the toilet waste pipe to the stack pipe, but run it along the wall by the head side of the bath.
Due to the size of the bath and room I can not move the bath vertically but could move it horizontally (i.e. towards the new toilet position).
Additionally, I also need to remove one of the basin wastes that goes into the stack pipe, but again I have no idea of the best way of doing this...
Does anybody have any ideas? I've done some very basic plumbing before but nothing remotely difficult.
Much appreciate it if anyone can spare some time to point me in the right direction.
Thanks a lot!
Aaron

I am glad I dont do plumbing ( or live anywhere like that either! )

It does not look like the cast iron soil pipes I am acustomed to either!

In fact it looks more like lead!

Tony

It`s galvanised steel note the patches of silver on it .....just like the galvy bath we used to have hanging on the wall @ home ......ex council flat built in the 60`s I`d point you in the direction of Kent, where a M8 who does plumbing might just travel to the smoke if you`re Sath of the river.........that`s on the premise that it`ll cost you a fortune for a plumber local to you

Cheers for your responses, it's ex housing association, but built in the 1880s. I suspect the stack pipe was put in in the 50s though.

How would you plumb the bath waste with the soil pipe in the way ?

The bath waste could be at the bottom of the (bottom) plan and run along the right side of the bath.

From speaking to a friend who has a bit of expereince they suggested cutting the galvanised steel soil pipe with an angle grinder close to the stack pipe and then connecting a new plastic soil pipe to this with a fernco coupling. The new plastic soil pipe would have a couple of 135degree bends in it to take it close to the wall at the top of the plan.

Does this sound do-able? Has anyone any experience of cutting galvanised steel soil pipes and attaching plastic soil pipes to them?

Whats the chances of me knackering the stack pipe from the vibrations of cutting the soil pipe?

Once again, much appreciate any words of wisdom.

Cheers,
Aaron

Obvious but make sure no one in one of the 5 flats above flushes the toilet, empties the bath, ect

Final Update... In the end I cut the soil pipe back with an angle grinder, fed a flexible pan waste connector into it, and connected it up to the new waste pipe that came from the new toilet. The stack pipe and soil pipe withstood the vibrations and nothing collapsed. Thanks everyone for your help.