Check your radiator fan is working, easiest way to do this is unplug the connector at the bottom passenger side of the radiator, and bridging the pins on the plug. The fan should come on slow & fast speeds. Then run the engine till the temp gets to just over half way, the fan should then cut in at slow speed. If not, the sender in the rad is buggered!
It may be your waterpump has failed, when you rev the engine do you notice coolant flowing out of the coolant return at the top of the header tank, where the thin hose connects?