Hi
Normally the clutch fan is the reason why most TCR 10,11 Taragos overheat because the of wear and tear the RPMs slow down over time and cant maintain the manufacturers RPM specifications to keep the radiator temperature regulated and as a result the cooling system over heats. (example the clutch fan might be designed to spin the fan 1000 times a minute but after years of wear and tear it might only spin 300-500 times a minute hence the failure to maintain the regulated cooling temperature and the result of over heating problems)
I have replaced my TCR 10 clutch fan to regulate and maintain the cooling system, however i have also seen water boil in a many a reservoir/water bottle and this is normally caused by a break in the head gasket near a cylinder, and what happens is when the piston travels up and down in the cylinder the air compression or carbon monoxide or exhaust fumes in the cylinder escapes through the leak/break in the gasket and into the cooling system and the water heats up via the pistons repeated function and like a volcano the pressure looks for a place to escape which is the water reservoir/bottle and this is why the water is boiling and ends up here.
The more you drive the vehicle in this condition the worse the damage will be to the cylinder head and components, the reason why you didn't pick up any water in the oil or through the spark plug combustion chamber is because the break did not occur near a the spark plug water or oil jacket, but boiling water in the reservoir is a dead give away.
There is another test you can perform to detect carbon monoxide or exhaust fumes in the water or cooling system and that can be done by buying a head gasket test kit where you remove the radiator cap place the tube with blue chemical in the place of the radiator cap and turn on the engine and let it run and if there is carbon monoxide or exhaust fumes in the water the chemical will turn green indicating carbon monoxide or exhaust fumes are present confirming a head gasket break/leak.
You need to reseal the head gasket if the break is small enough or change the head gasket and get the head machined etc.
Just a note if the blue chemical turns green instantly or really quick when you use the head gasket test kit it means the break is to big or server to reseal and needs to have have the gasket changed and cylinder head machined.
Cheers