In preparation for replacing the basin and washing machine waste pipe
outdoors run, for connection into the toilet soil connection, I was
delighted to find an "antivac" bottle trap in B&Q ... meaning to do
the full job next weekend, I fitted this in preparation - at least
I'll stop the washing machine gurgle, I thought to myself.

Now, there is a distinct drain smell in the cloak room ... so clearly
something is amiss. Is there any trick with these traps (no
instructions on bag) or is it just (a) crap design, or (b) just my
luck - faulty ?