Last night Martin discovered that water was running down the side of the wall in the hall. On further inspection we noticed that the ceiling was very damp as well. We traced the water upstairs to discover that the source was the toilet beside my room. For a horrible moment we thought it was all coming from the pipe downstream of the u-bend but on closer inspection we realised that it was coming from just upstream of the cistern. On the bright side this means that I haven't effectively been urinating on the walls.
As an engineer who deals with flood modelling software it occurred to me that I could have built a computer model of the flood. But I had to call a management consultant to tell me how to fix it.