We have noticed that there is a smell of rotten eggs in the down stairs loo of my son's house. The smell seems to arise when there is more usage (he works away). The toilet was installed 2 years ago and the smell intermittent during this time. All new plumbing was put in at that time. The waste, having joined the waste pipe from the ensuite, goes out to the sewer. We think the smell is from the toilet rather than the sink or shower. Any comments / advice please?