There IS a TSB on the Zetec for the temp gauge reading higher than it should when the temp is in fact normal.
The correct way to check for this (I'm an ASE master tech), is to take it to a shop that has good tool selection, and have them use an infrared temp gun to read temp at the water outlet of the engine (before it hits the radiator), and make sure that temp is within spec while your gauge is sitting at the 'L' like it was when you thought it was running hot. Once you verify the actual running temp then you can figure out what to do from there. If it WAS just the gauge reading hotter than it really is, then I'd probably take apart the instrument cluster (not too hard) and remove the needle for the temp gauge and put it back on in the spot where it read correctly while I was frequently checking it with the IR temp gun. Or you could just learn the temps in your mind that the engine is really at at various gauge readings. To me, it's not as important what the gauge actually reads, but just whether the gauge is reading outside the area where it always sits at while you noramlly drive the car. For instance, if it always reads 'A' to 'L' everyday while you drive it, and then one day it reads past that, THEN I might suspect some problem with engine cooling. Obviously, it may run a few degrees warmer in the summertime, but you catch my drift, right?
That's just my 2 cents.
