Thanks very much for all your comments Fred, they were useful. I have
a few more questions in response

I think the only way to make a reliable connection is to use a boss. If
that is the case, then you need to make room to fit one, even if that
means hiring an SDS blaster to make room to fit one. You can make good
the render/brickwork after the even.

OK I'll have to have a good look to see whether this is feasible in
the space I have to work with - it's very tight.

you don't really want a right angle bend in the pipe
just before joining the stack

Why is that? I understand that bends in general should be swept bends
and not right angle, but what's the significance of proximity to the
stack exactly?

so maybe that means working from inside
the house & removing bricks until you can make a straight connection to
the stack.

This could be interesting - I have a fitted kitchen on the other side
of the wall! The soil stack may just coincide with the Dishwasher bay
though.

Also, why copper, it's not the normal way to do things so as
a result there won't be fittings to do the job. Suggest you do it in
waste pipe of the appropriate size and

I understand that using 22mm copper is the best way of plumbing
Saniflow waste (as it is more robust than solvent-weld plastic). I
don't think it's that uncommon, therefore I assumed that someone may
have come across the same problem
- joining 22mm copper to cast iron stack

to make a copper to (40mm plastic) waste pipe joint

Are there fittings to join 22mm copper to say 40mm plastic waste?