Great idea re cling film John, I should have thought of that, same as you I flush plenty of water down the soil pipe even fill the sinks and pull the plug at the same time as flushing to give a good sweep through the pipe. I've seen these water computers attached to outside taps to water gardens in owners a scene, don't trust these things as they are made of plastic and won't withstand high mains pressure, I use one at home and came back to find it leaking. Best option in Spain is both meter valves off.