I think I have a blockage. When I run the cold tap fast into the kitchen sink, an unplesant toilet smell comes up form the sink plughole. The smell is not there unless water is run in the sink. Its all new pipes under the sink as I recently had a dishwasher fitted. The sink waste pipe drains into the soil stack from the bathroom above. Anyone know how/why the toilet smells are coming up the sink wastepipe and where the blockage might be? Would ordinary drain unblocker put down the kitchen sink reach the blockage?