The age and number of the pipes. It's easy to underestimate the scale of the problem. There's been a replacement process going on in London to replace ancient

The age and number of the pipes. It's easy to underestimate the scale of the problem. There's been a replacement process going on in London to replace ancient cast iron and wooden box piping on the mains supply for the past few years. That's mostly Victorian and earlier installations, but there isn't anything like a full inventory of where they are, and sometimes what they find is that the original pipe has completely disintegrated: the water is just flowing through the void in the clay. I do know of one case (not in London) where complaints about the quality of a house's water led to the discovery that it was, via a somewhat convoluted plumbing route, still drawing water from the nearby river via Roman pipework. If you can find a leak, stopping it is usually straightforward, but on a colossal, heterogeneous and patchily documented pipe network like the UK's, the best you can do is patch it up as you go along.