Michael Chare wrote:

Is there a little box that I could buy which would have an Ethernet port (and
IP) that I could attach to my home network and would allow me to use my computer
to monitor the status of the mains voltages circuits (of my central heating
system)?

Most of the "smart" UPSes can monitor mains voltage, and have the added
benefit of keeping your PC going even when it reaches zero volts :-)