Setting up a home sensor network

Every year or so I have the (unoriginal) idea of monitoring temperature in my flat to work out if my heating is throwing money away by coming on at the wrong times. And in the last few years various home temperature/heating monitoring systems have become available to do this:

Market Summary (as of Apr 2016)

Product Price Pro Con Notes for my usage
Nest £249 Looks good No external sensors (one room only) Doesn't work for me
Hive Doesn't work for me
Honeywell Lyric
Honeywell Evohome £249 + £50 Looks good Ludicrously expensive Not worth it & doesn't work for me
EcoBee Doesn't work for me
Apple Homekit It's Apple & doesn't work for me
Netatmo Doesn't work for me

So, not much point buying one of those. But really all I want is a temperature sensor + wifi/bluetooth on a cheap chip and some software that shows the details on a pretty graph. I know all of these things are doable, but no-one seems to produce a product that brings them all together well. The best option I can see at the moment would be a Raspberry Pi Zero + temp sensor + bluetooth chip, so around £10 per sensor; but then it would require a power supply (battery power draw is still rather high so would need recharging daily, see )

Equally, I can't find a pretty open source sensor monitoring system that takes in different sensor readings (temp, humidity, pollution etc) that displays them nicely. The best one so far seems to be: but it's a bit of a beast (in the nicest possible way).

Commercials systems to do this similiar things include:

Product Price Notes
Sense by Hello $129 each Multiple sensors for temp, pollution, light, noise, intended mainly for bedroom to monitor sleep

The search continues... (will update this post as and when)