My Antifreeze is something I need to address when the weather gets a bit better. Mine is reddish brown and can’t see the needle readings on my antifreeze tester. So for piece of mind I syphoned 5 litres out and replaced with 5 litres of holts 05 hoat to get me through the winter. My thermostat is working fine but like @@RichardM I have a Renault in-line thermostat in my toolbox just in case. As for the starting, I only have bother below 5 degrees C. And the it only takes a few more seconds to start than normal. (Must have some decent compression). I have found that all my glow plugs are duff, I have brought a new set, but again will wait for the better weather to change.