Thermostat will need changing. Its not far off the correct values but its still low.
Might squeeze another 1-2mpg more.
Sounds daft but have you checked your tyre pressure too?
If you have had new Maf, filters and a service so everything is as clean as it can be then I would really start to think the problem lies else where.
I'm not so converse with the diesels but making sure the exahsut isn't blocked or blowing or Lamda sensors are not reading incorrect could be the next step.
16v Diesels are not that economical, but you get alot more hp and torque than the 8v. An subsquently you use the higher rev range. I'd still expect low 40's though at those speeds.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts