@nayffy This is definitely something we've observed as well. All temp sensors have errors.
My suggestion would be to pick 1 and increase or decrease your W/C until you're comfortable. 

(I know this doesn't answer your question about getting an accurate sensor)

The best way to handle multiple thermistors/thermocouples! Thermostats indeed any temperature sensor is to do a “cluster test”. Basically you put them all in the same location then allow them to stabilise for a while and then record the values. Calculate the average value of all the readings and select the one nearest to the average as the “true” value. You can buy more accurate temperature indicators but you don’t need one, you simply need to know which one to use as “true”. All sensors have some uncertainty and the indicated value often doesn’t actually matter. Don’t fall in the trap of believing that one which shows more decimal places is more accurate. One meter with one decimal place which is close to true is better than 3 decimal places of apparent (but wrong) accuracy.

Posted by: @hcas

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@nayffy This is definitely something we've observed as well. All temp sensors have errors.
My suggestion would be to pick 1 and increase or decrease your W/C until you're comfortable. 

(I know this doesn't answer your question about getting an accurate sensor)

Thanks for the answer, with W/C I'm basically judging on whether I'm comfortable in the clothes I'd normally wear around the house or not. I still have some minor adjustments to make but I'm not really using the thermostats too judge that as 22C in one room can feel freezing compared to 22C in another for some reason!

Posted by: @judith

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The best way to handle multiple thermistors/thermocouples! Thermostats indeed any temperature sensor is to do a “cluster test”. Basically you put them all in the same location then allow them to stabilise for a while and then record the values. Calculate the average value of all the readings and select the one nearest to the average as the “true” value. You can buy more accurate temperature indicators but you don’t need one, you simply need to know which one to use as “true”. All sensors have some uncertainty and the indicated value often doesn’t actually matter. Don’t fall in the trap of believing that one which shows more decimal places is more accurate. One meter with one decimal place which is close to true is better than 3 decimal places of apparent (but wrong) accuracy.

A great answer! Unfortunately all the thermostats are hardwired and have no batteries so I'm unable to get them together. I was just hoping there was a reliable thermostat that didn't break the bank that I could use to calibrate the rest of them. I know the neostats are prone to "drifting" and also not very accurate, but I can't afford to replace all 7 of them especially when I don't use them to control the heating, only to satisfy my curiosity of the current temperature.