Roger, thanks for your detailed answers. It should not ne too difficult when I cut some cardboard to reduce the efficiency of the aluminium radiator, which consequently to your thoughts should help to lower the water temperature of the engine.
I have understood the consequences and effects but I haven't really fully understood how it works and how the connection between the warm bypass water and the cooler radiator water come together.
I assume that both water hoses meet before the thermostat. But how is the mix determined, and why does too little cold water seem to come into the circuit towards the engine behind the stat when the new radiator cools so well, better than the old smaller plastic radiator? But the engine runs hotter...
Your cardboard experiment will be interesting.
I don’t know the detail of the Sigma so can only attempt an explanation on the Duratec.
Below is a pic of the Duratec showing the coolant paths and how they mix at the stat housing. Also a pic of my old 90deg stat. Note the stat is a double valve arrangement. The primary valve is pretty standard. The secondary valve is the plunger at the end that seals against a seat in the block. When the primary valve opens to let the rad coolant into the block the secondary valve closes off the hot by-pass coolant.
As said before it looks like it is dependent on a precarious temperature balance between the hot and cooled coolants that might hold the answer. Just a wacky theory……