Having tried both capacitive and resistive moisture sensors, I found the resistive much more stable and accurate and then spent a bit of time calibrating it against changes in temperature:

The blue line shows how the moisture readings change with temperature and it appears to be a straight line. In reality, it tapers off as temperatures rise above 50 degrees C. I used a very simple formula: 

moisture = moisture - (moisture * soilTemp * 0.065)

by trial and error to get the red line. As can be seen, it's not a flat line so the cobbled formula could be refined by incorporating a cobbled sine function, but accuracy to +- 2% moistre is absolutely fine foe me at least!

Calibration files for the weather station are available here: https://github.com/paddygoat/Weather-Station/tree/master/Calibration%20calculations

I also fixed some of the graphs on the GUI so that, among other functions, soil moisture and temperature can be viewed side by side: http://www.goatindustries.co.uk/weather2/soilMoistureTemp01.html to check that the calibration is going to work. It should display a steady decline, irrespective of temperature, until it rains again.