I want to add a new sender to my 200tdi engine to send a signal to a VDO gauge that i added. I want to leave the factory sender in place that signals the OEM dash gauge. I've heard the threads are an odd size anyway.

So where is the best place to drill and tap a new hole? I have a 1/8" sender, so i was thinking i would just drill and tap a new hole on the thermostat housing.
I want it in a place that was not contingent on the Thermostat being opened/closed.
Has anyone tapped at this location? see photo below.

i would wield a nut into the long steel heater hose on the head

Not long after i posted this, my OEM sender and/or my temp gauge went out. It was not registering anything. I checked grounds, and leads. Nothing.
So i ended up drilling through the original sender, and tapping for my 1/8" NPT VDO sender that i got with my VDO temp gauge.

HOWEVER, not matter the ambient temperature, my gauge reads 210-215 F. (200tdi Disco in the States). This seems high. My thermostat is opening (upper hose hot) and i just replaced by viscous fan clutch.
Thoughts?

well the thermostat should start to open at 82°C (180°F) and probably be fully open at 92°C (198°F) - I think.

According to your new gauge it's operating at around 100°C - that does seem a little too hot.

You could try a new thermostat.