On inspection (throught rodding eye) the plastic soil pipe stops 10mm short of (i.e. it doesn't fit into) the clay pipe, as I would have expected. Therefore the join is relying on the seal between the OD of the pipes and the ID of the plastic joining sleeve that both pipes fit into. The leak is between the clay pipe OD and the joining sleeve ID.
Is this normal, or should the plastic pipe go slightly into the clay pipe?
If so, unfortunatly, dropping/replacing with a longer plastic pipe is not really an option. Major distruction to bathroom tiles would be required.