Double Dark Side! Diesel & 8P
bit strange mate.
Does the water temp ever reach 90 degrees on the inside guage? If not I would unplug the fans to stop them kicking in and then see if the temp actually reaches 90. If it does then you know the stat is opening properly and isn't at fault.
In VCDS you can check all temps and see when fans are kicking in and things like that which would be helpful if your mechanic has VCDS.
The fans shouldn't kick in until 100 degrees so maybe you have a dodgy fan switch!