On 26 July, 12:22, Michael D wrote:
It's an interceptor trap:

http://www.draindomain.com/interceptor%20traps.html

Ahhhhh - that explains it. *Many thanks for the swift responses.

One other question: how should I re-attach the cap? *If I just push it
back in it probably won't be a very good fit & may fall out again.
Should it be fully or partially mortared in to prevent smells getting
back through? *I don't know if something more modern like mastic may
work as it would make it easy to get out.

Michael

I wouldn't bother with the cap. AIUI, the modern solution would be
straight-through with no trap at all (unless there was a special
requirement like grease-traps for restaurants). I would do my best to
clear out the trap and get everything flowing as freely as possible
though.