A propos of @derwendolly 's experience, at a house I lived in in 1997 I replaced the lead pipe from the stopcock in the street to the house with a blue MDPE. I tunneled under the pavement a couple of feet to reach the fitting on the stopcock and cleared out enough earth to swing a spanner and felt very pleased with myself. However, there was a small utility room and toilet out the back whose water seemed to come from a different source. I'm not sure where, because it didn't seem to be slowed down if I turned off any of the stopcocks I could find. I moved out before it became a problem so I didn't need to do anything drastic like hammer the lead pipe flat. Sometimes when people add to existing systems or introduce new sources of supply you get these kinds of anomalies. I think when the houses were built in the 19th century they'd had a supply down the back for the toilets but no running water inside the houses, and then a 'new' pipe was put along the pavement for the houses themselves, when people were starting to want indoor plumbing.
