ENGINE TEMPERATURE ISSUE. 1.9 TDI.

The engine is taking a long time to warm up (very much longer than it used to be ) and when it does warm up whilst initially the temperature needle seems to sit at normal operating temperature it will quite readily move up the scale towards the red line and one has to be very careful in deed in the speed/engine management.  My initial interpretation is that the thermostat may need replacing but is this where the issue is pointing please?  I know that water pumps on these engines. Is this likely to be part of the issue.

Thanks in advance for any helpful comments or suggestions.