Engine temperature

I'm fretting about this one again. The car sounds and runs well enough, but I still feel fuel consumption is higher than it ought to be for the car and the way I use it. This isn't about short runs - I don't take it anywhere for less than a 20-mile round trip - or any of the other bleedin'-obvious factors. It had a new air filter a year ago, which hasn't made any noticeable difference, so I'm back to temperature.

If I break my fuel records into 12-month blocks to remove any seasonal effects, I see mpg averages of 33.3, 32.9, 34.2 and most recently (after the air filter change) 33.1.  Not disastrous but not what I'd expect from a modern diesel engine, well maintained and fairly gently driven.

What I still don't see from the gauge or the app is the coolant or oil temperature reaching the high-80s or low 90s I I might expect. Yesterday's 19 miles each way to Bracknell (36 minutes each way, half of it at or near motorway speed) shows only one peak above 80°C and the rest bobbling about in the low 70s. This can't be healthy and may have consequences for temperature-sensitive bits of the emission control system, such as the particulate filter, so I think it's time to get the thermostat checked out and probably replaced.  It's presumably the original component from 2013, so it's arguably done well enough. I might also have done better to replace it when I first suspected something was up, but that doesn't matter now.

33/34 mpg seems pretty low? I know yours has a larger engine but it’s a country mile off the 50+mpg I get from the 2.1.
Actually saw an average of 62mpg on a long run last week and that was with the (empty) bike racks on top and north of 190,000 on the odometer. No special fuels either, it lives on a diet of Sainsburys or Morrisons cheapest.
Drops a bit on short runs or with the bikes on top, but I’ve never seen low 30s like that. 42mpg or thereabouts if absolutely caning it fully laden and enbiked.

No dramatically different from what I would have got I reckon on the sort of runs you mention.

I am trying to work out what changing the thermostat would do. Presumably you think it is opening the feed to the radiator too early, keeping the engine too cool and therefore injecting a bit more fuel than is necessary. But if this was the case, wouldn't the emmissions be an issue?