MK2 mondeo 2.0 16v coolant temp

[NST]

My father in law has a small problem with his A1 condition '99 MK2 Mondeo 2.0 16v (which I will take of hands when he comes to sell!). I noticed a couple of weeks ago that the engine temp never really gets half way up the gauge. whilst sitting in traffic it slowly got up to half way, but once moving around town and motorway speeds the needle crept back to just over into the 'normal range' about 1/4 up the gauge.

I thought it might be the thermostat, but before i go and replace it for him, i would like to make sure it might not be something little more serious.

Any ideas?

NST