We have a 1930's semi which has an iron sewer down pipe, which is
cemented into a ceramic elbow which is in turn concreted into the
ground.

The collar of the ceramic pipe (into which the down pipe is fixed and
which is above ground) appears to have fractured all the way around
where it meets the rest of the ceramic pipe.

This means that when water and 'other liquids' run down the inside of
the down pipe, they leak out of the pipe where the fracture is.

The ideal solution would be to dig the old pipe out and replace it with
plastic I guess, but it's set in concrete.

Is there any way I can seal the crack? If I surround the outside of the
pipe with cement, would it seal it?

Would that expanding foam be a better bet, or perhaps I could use
bathroom sealant?

The fracture is quite wide - perhaps 3-4mm. It looks as though the
buried bit has sunk slightly.

--
Richard Markham