Yup. Upstair bog filled to the brim..taking an hour to empty, filthy,
stinking..tried a bit of caustic overnight..no joy. Tried coat hangers,
no joy. Basin/bath coupled in to vertical stack below bog all fine..must
be U bend or where the pipe joins the stack..utterly inaccessible..all
inside the wall..

Hmm. Normally would stick hose up it but danger of exploding crap heap
on bathroom floor. Cant get to stack top from below. Bugger.

Well hose is a good rodding agent anyway, saw off bit of scrap hose (the
one SHE left in the frost with the tap turned on, that split to buggery)
and push it round u bend and wiggle. No joy. Should I siphon te ****
out? no way.

Well at least lets see where the hose is and the blockage by blowing
down hose..and being careful not to suck. Hmm. That's ODD. No bubbles.
so how come the air is getting out, but not the water?.

Blow some more and suddenly its clear!

I can't fathom it personally, but it worked.

Maybe someone else can.

AFAICR the stack is pretty much straight up and down, with a right angle
at the bottom that runs into a manhole covered thing. Forget the name.
There are various other things coming in at lower levels..the kitchen
waste system goes into a collar there, and at bathroom height the bog
waste couples in, with the basin/bath sharing a final exit into a
collar. Above that lot the stack goes up and a reducer takes it to 3"
pipe, which wanders off through the roof space to exit under the eaves
way down..

So the blockage - and again I think there was nothing abnormal except
maybe a plastic comb or something silly - down there, must have been
somewhere where the bog pipe joins the stack, but why would blowing down
it work, when a pan full of ****ty water didn't?