So, given that I only want indicative watt readings, my mains voltage and frequency is stable enough to consider variation negligible and I don't need apparent/real power or power factor, I would only need the current clamp transformer?
That only leaves the trouble of powering the monitor in the meter box where there is no socket. A Wifi enabled MCU uses about 50mA, so it won't run longer than a few days on small batteries. Even if I go for low power, sleep mode and only sample and send the data once a minute it's still going to need it's batteries changed every few weeks.
The existing proprietary meter lasts 3 months on 2 AAA cells and even that's annoying.
I can't put the meter in the apartment without hacking into the consumer unit, as the cabling is all concealed behind a wooden panel.... I rent. I wlil double check there is no way to access the live feed line.