AUZ 1.8T -

At the last MOT the tech noticed that the temp. guage was reading 10 degrees lower than the actual engine temp, and more recently the temp warning symbol has been coming up despite the car not overheating (eg it'll beep as soon as I start it up in the morning). It used to be intermitant, but now it's plain annoying!
It's clearly either the temp sensor or the temp warning sender. Any ideas?

Also, what exactly is the difference between the blue and the green sensors?