It's a combined drain until it reaches the public sewer which is usually in the road. Everyone on the line is responsible for its upkeep etc. If there is 6 houses on the line and the blockage occurs after the last house, before it enters the public sewer, then everyone is responsible for the clearance. If it occurs two houses away from the end of the line i.e. the two houses nearest the public sewer are unaffected then the other four houses share the cost of clearance. This also applies to repairs. Your county council drainage section should be abel to tell you who is on the line and which way it runs. If people refuse to contribute then they will serve notices on all the houses involved and teke it into court if necessary. These situations can be very stressful especially if one of the neighbours is in denial. .