Oh joy, oh joy, Thames Water have buggered up something big time, most London E postcodes haven't had any mains water since ~16:20. That's got to be something really major, power loss or worse at a major pumping station. My guesstimate is that it's got to be affecting ~500,000 people based on the list of postcodes they were showing on their status page earlier, they've since updated it to just say "East London".
Thank god for paranoia. I turned on a tap at ~16:15 and got a trickle (< 2 litres/min) before I knew there was a major problem and had the good sense to use it to quickly fill up an empty 5L bottle I keep handy for just such an event. At least I've got drinking water.