It is possible that the soil to clay adaptor has moved or been damaged.

There is an application detail drawing showing how the connection should
look he -

http://www.hepworthdrainage.co.uk/pd..._details_2.pdf

Thanks for this. I see the plastic pipe doesn't go inside the clay pipe
(although this would surely make the joint better and not rely on the
'rubber' seals). Of course the plastic pipe would need to be a smaller
diameter.

My adaptor hasn't moved (permanently), I think the seal has just failed
around the clay pipe. Maybe the plastic pipe was moved up and down slightly
when I had a new toilet fitted a couple of years ago and the adaptor moved
with it, breaking the 28 year old seal around the clay pipe.

I'll call the advice line.

Many thanks,

Darren.