I need some advice. We had a new bathroom fitted a year ago and have been plagued by an awful smell since. If we flush the loo or use the bath, and run the sink taps straight after, the whole house fills with an awful smell. We have had a drain specialist out twice who snaked and found nothing, and several plumbers, who have put non return traps in, but nothing has helped. I know its not poor hygiene, we never had this problem before the bathroom was changed, but it started immediately after the new bathroom was fitted. The smell seems to come from the sink drain and from behind the toilet.
Any advice greatly appreciated. The house is rented and I am sure my housing association thinks I am making it up! They have ignored my last two requests to sort it out, I am guessing on the grounds that nothing has been found by now.

Sounds like a venting problem. Immediately after flushing or using the shower, slowly pour about a pint of water down the sink. See if that changes anything.