Cool running engine

[dieselj]

I've had my berlingo 2.0 hdi a year now.
During this time, the temp gauge has never been quick to get to 90 degrees compared to previous vehicles I've had. I can drive round town for a couple of hours and it never gets above 70 degrees. The only time it gets near 90 is if I leave it on tick over for a good 15 minutes or when motorway driving.
I'm pretty sure fuel economy isn't as good as it should be around town, probably 35 mpg at best. Could the cool running engine be the cause for poor fuel economy?
I'm suspicious that the thermostat may be stuck open?

have you changed the thermostat? its simple enough to do and made a difference to my 2.0hdi.