Up until today the temp gauge has never ever ever gone much above 90 even when sat in the most ridiculous of traffic jams, however today this happened.

Sat stationary and the temp gradually rose to 100
Fan kicked in as normal at about 95 but didn't seem to affect the indicated temperature like it has before (just over 90, kicks in the cools right back down to just under 90)

Points to note are

When i set off the gauge sank pretty much instantly to its normal operating temp or what i've come to know as normal at say 85-87 on the gauge

Coolant level is bob on

its not mixing oil and water nor using any oil or water

radiator was uniformly hot

I do however have a small water leak from the left hand heater matrix pipe and have had for a while, could this have caused an air lock maybe? Could the gauge be playing funny buggers?