I'm slowly losing coolant, 91k miles on it.  I've been keeping a close eye looking for leaks and can find nothing obvious and there is no steam from the exhaust.  The oil cap is clean from any white gunk.

I'm fairly sure it isn't due to air in the system, I topped it up 2 weeks ago and in that time it's gone from max to min line.

i had fiesta 1.6tdci which leaked coolant slowly from black plastic thermostat cover (on end of cylinder head - passenger side end).

i only found where the leak was by putting radweld in it. radweld is Red. this left a thin red stain coming downwards from the thermostat cover which I could only spot from underneath the car looking upwards.  The bolts were still tight. I assume it was because the plastic had deformed. The car was less than three years old. 

But I also understand that many fords have suffered from microcracking of the coolant reservoir.

6 hours ago, fordification said:

I'm slowly losing coolant, 91k miles on it.  I've been keeping a close eye looking for leaks and can find nothing obvious and there is no steam from the exhaust.  The oil cap is clean from any white gunk.

I'm fairly sure it isn't due to air in the system, I topped it up 2 weeks ago and in that time it's gone from max to min line.

Have you bled the system of air to be sure? isetta is right , they seem to have a problem with cracking expansion tanks.  Can you put a piece of card or something under the tank and see if it gets any coolant on it?   Check the cap as well that .....