Got a bit of a problem with a leaking soil pipe and would appreciate
some advice...

I have a boxed-in, steel (not cast iron) soil pipe, which feeds into
what I assume to be the top of a clay drain pipe at ground level (ie,
inside the house). Sort of like this...


steel soil pipe
| |
| |
| | | |
-----|__|_ _|__|------------ground level
| |
| |
| |
| |
| | clay drain


The gap/interface between the steel and clay pipes had been plugged and
sealed with mortar, but this had cracked (radially), and there's been
quite a bit of slow leakage within the boxing over time (fair bit of wet
rot).

I've tried to gently chip out the mortar 'interface' in order to reseal
it with new mortar, but not unsurprisingly have managed to bust the clay
pipe. That is itself embedded in concrete - see
http://tinyurl.com/ap2fg for a photo of the current situation! (there's
also a new crack now, just visible at the 3 o'clock position).

At this point I really don't know what the hell to do. Digging out the
broken clay pipe - which is embedded in the solid floor and then
presumably turns to pass through the foundations - looks a non starter,
and the only thing I can think of is just to lather the whole broken
joint in oodles of wet mortar and hope for the best. Is that likely to
work, though? It would make matters a lot worse if it doesn't! If
that's the only way, any top tips to ensure a waterproof seal?

--
Thanks
David