Recently purchased a 2007 Mondeo 2.0 TDCI as a bit of a stop gap, only  82k on the clock, full history and well looked after for its age. 

Car seems to be driving perfectly well, However I noticed that the needle for the temp gauge seems to fluctuate as I'm driving. If I let the car idle or drive round town its getting to around 80/90C, However as soon as I get on the motorway and I'm in higher gear, its slowly dropping back down to 70C, I've had the Diagnostic screen on the dash to check it isn't just the dial that's faulty and its consistent on there too. What's the typical operating temp on these, and what's likely to be the cause? I was thinking possibly the stat?

Once warmed up it should be at a consistent 90°C. I suspect that your thermostat is not fully closing so it is overcooling at higher speeds. Replacing the thermostat should be a relatively cheap and easy job.