In article ,
Andrew May writes:
Andrew Gabriel wrote:

You might be describing an interceptor trap, but difficult to tell
without a picture.

That soundes like it. Almost exactly matches the first picture on the
second set of pages he

http://www.plumbinghelps.co.uk/page2.htm

There's one in a 1900 manhole here, probably exactly the
same (except I can only see the exposed bit in the manhole,
and it's made of iron).

I'd hate to have to remove the inspection plug -- probably
never been removed in 100 years.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]