I have noticed a smell of sewage in my downstairs toilet. It only seems to happen when the shower/bath in the upstairs bathroom is used. The water from the shower flows into a small 'drain' right by the outside wall of the downstairs toilet, next to the soil pipe. I have noticed that this 'drain' seems to be fuller than it has been in the past and takes a while to drain away. Is this connected to the smell in the toilet? There is no detectable odour coming from the manhole cover or the drain - it only seems to be in the house. Sorry if this seems obvious - I am a girl! 
