Nissan Micra faulty temperature gauge

The temperature gauge on my P reg micra shoots sky-high when I travel at about 40mph+. At a steady speed it moves up and down near the top limit. I don't get any warning lights and the engine is not over heating because the fan doesn't turn on and it doesn't feel hot after driving. I am just curious as to what the matter is and how to fix it, if any one can help

Could well be a duff sender. Cheap and easy to fix.
It's probably worth changing the thermostat and giving the system a flush through and refill with antifreeze mix.

Great stuff!

Cheers ^neo^, i'll give it a go!