Lobster wrote:
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).

It's difficult to get cement out of the socket, as you've found. The
bedt way is very gently with a gentle hammer drill and a small bit,
making lots and lots of holes, and drilling at an angle. However.

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?

You need to get the metal pipe out of the way, & push in an adaptor
which you can get from a BM or PM. It's a plastic version of the
clay one, with rubber fins which seal between the clay & plastic.
Break off the rest of the socket as cleanly as you can, then push
the adaptor in. Re-position the metal pipe. Fill the socket with
cement as normal.