Is it sewage or not? If it's what normally goes down toilets then you need to knock on their door and tell them and that you happened to notice it from upstairs. Their problem could end up being your problem if it is not sorted soon! As I already said the council will come (for FREE) and look and hopefully fix if its just a blockage. if its a digging job they will charge for the labour and replacement pipe - that s if the problem lies in the garden. They will say that it is the property of the householder. Anything from where it joins the main sewer (usually outside the garden) is the council responsibility to fix and you will not be charged. I know all this 'cos I have been down this road. It cost about E1,800 divided between 6 houses in my case, but this involved getting planning permission, plant hire, digging up concrete, replacing a pipe and paying two men. All done in half a day, no problems since. Expensive, but it had to be done!