GregMN - Have you used any "Bar's Leak" recently? No
Your radiator is blocked, either externally or internally. I agree but have not inspected for external blockage in detail.

engatwork - It is either an air flow or coolant flow issue. Airflow seems to be ok. The viscous fan moves freely when cold, but stiff when hot (that is too funny!) The elec fan works fine. Sometimes the temp indication jumps a bit. Usually it is stable 95C. Maybe the radiator has some internal blockage and the flow is being disrupted?
I have not read in enough detail the history why the thermostat was replaced. I'm an idiot!
Was it cooling ok prior to the thermostat getting changed? YES and NO - YES - the high temp came on suddendly. I was lucky to notice.
NO - the engine was running cooler before the summer. No doubt. Around 80C, now at 95C.

Olivier - Did you look at the coolant again? Three times since driving about 20 miles. I think it is ok. I dont know where it should be. It is filled just above the mark when looking straight down into the filler hole of the reservoir. Is that ok?