I am using a RF-tech 433MHz wireless thermometer purchased for about £25, and may be purchased
here.
There have been a few minor problems
with this, the batteries can give out with little warning and occaisonally the transmitter can play up when it gets damp,
however generally speaking I am pretty pleased with it.
As the only suitable position for the sensor is to the side of the house that faces almost due south, finding an arrangement
that protected from direct sun was a little tricky. Unfortunately simply placing it 'in the shade' wasn't good enough.
After some experimentation I devised a shelter that provided ample protection from solar radiation, yet gave adequate
opportunity for reasonable air flow around the sensor.
I would certainly not claim that my readings are up to Met Office standards however compared to data from other nearby
sites they seem pretty good.