On Jul 9, 2:42*pm, "John" wrote:
"The entire pipe run will be sealed, so no trap needed inside."
Isn't this likely to cause siphoning?

No, the pipe will be draining into a gulley-style drain, so as there's
an air gap between the two, there's nothing to siphon.
I know this isn't "current regs", but the sink has drained into it for
the last 40-odd years or so, and it drains into a private septic tank
anyway (ie: it still goes to the foulwater drain, not a rain soakaway)
really I can't see the problem.

On Jul 9, 2:41 pm, RobertL wrote:

Would it help to use two 23mm pips in parallel to give the same flow
area as a 32mm pipe?

Hmmm, subtle idea, but even drilling one hole through the wall is
going to be enough aggro. If it has to come to it, I suppose it's an
option, but drilling through stone-wall is going to be bad enough as
it is.