I don't understand this - surely if you've got a thermostat then there should be no need to block off part of the radiator as water won't be flowing through it. If it gets hot enough to open the thermostat then the water will just cool more quickly and the thermostat will close again.
I have done several long journeys in the last week with the low ambient temperatures and the oil has been around 60 degrees and the water a steady 70 degrees. As soon as I slowed down, the water temperature rises fairly quickly and the thermostat opens to reduce it again. When stopped in traffic it rises to the point that the fan cuts in. This would all seem to be the designed behaviour. I think that the really low ambient temperatures just produce lots of cooling of the block and in fact the thermostat rarely opens when driving at speed.
The heater works even when the thermostat is not open so if your heater isn't providing any heat, then either it's broken or your thermostat is broken. Bear in mind that at speed, the heater is pretty ineffective anyway as the warm air is rapidly sucked out of the cockpit faster than the heater can replace it - when you are stationary or driving slowly it works fine.
Yellow SL #32