D3 3.0 tdi temp gauge / heaters issue

[Bushido]

Long and short folks ...

Temp gauge often sits at 70 degrees on runs. Will rise occasionally to 90 when sat in slow traffic / stationary. Cabin heaters are typically warm enough to take chill out but no more. Are hotter when gauge is sat at 90.

Replaced the thermostat and coolant temp sensor up to now with oem parts. No difference.

Where do we go next please? In dependent suggested back flush of coolant system.

Any advice appreciated