Ok with the temperature gauge that high the coolant should be boiling out of the header tank like a volcano, but the partly-warm radiator and steam from the tank disagree with the gauge so time to change the sensor.
Buy a genuine sensor from TPS (about £40) because aftermarket versions suck and the cheap 'genuine' ones on ebay are Chinesium fakes. Also buy the seal and clip because they'll need changing too. You can do it without losing much coolant as long as the cap is still on the tank. The sensor is down the back of the engine under the air intake tube. Its likely to be well stuck in there and there will probably be lots of schmutz in the hole - clean that out with a screwdriver and then a small wire brush or stainless pan scrubbing thing. Lubricate the new seal with a smear of washing up liquid and it should go back in without a fight.
059919501A dual temperature sensor
032121142 retaining clip
N90316802 o-ring seal
Opps missed your post Andrew!
