I may be wrong but is there not a coolant sensor which drives the gauge and another that sends info to the ecu to control the mixture?
Did you change the right one?
As I said, I may be completely wrong but at least I tried to be helpful!
Blue for the dash gauge, brown for the ECU (black on the later cars).
The pscan will show what the ECU sensor is reading.
My mk1 VVC failed one MOT on emissions due to a slight leak on the exhaust manifold to down pipe joint, I had changed the exhaust the MOT before and asked the garage to nib up the bolts on this joint but they hadn't allowing the gasket to burn through.