[Lord of Hog]

If it did it after a period of crawling in traffic or after rapid deceleration (for example coming off a motorway into a queue) then the most likely culprit is your fan, followed by your water pump. Test the fan as per the advice offered earlier. Dying water pumps often (but not always) leak onto the outside of the engine. Also allow the temperature to build a little and then see if adding a few revs makes it stabilise/reduce.

If the temperature has been getting a little higher than normal over a period of time then you've got a leak. As the others have said, take off the acoustic cover and have a look/sniff in the V for evidence of congealed coolant around the thermostat housing and solid pipes. Look for the tell-tale pink stains at the attachment points on your hoses too. And of course the good old water pump.

If it suddenly decided to spike after behaving itself before then you may well have an air lock. Has the system been drained and re-filled recently? Or have you done anything with the heater matrix?

The good news is that, with the exception of the water pump, everything is reasonably easy to fix. (And even the water pump isn't THAT hard if you've got the right kit).

See if Simon (SD1too) is floating around as he's normally pretty good at asking the right questions in overheat situations.