To flush it properly, you have to take the thermostat out. Antifreeze/water must be 50/50, to stop boiling, evidently the Windsor has a couple of hot-spots in the head, so you need 50/50 to prevent localised boiling.
If you can't be bothered removing the thermo, just drain, refill with water, and drive around a bit. Do this a couple of times, although if the radiator hasn't been flushed for a while it is worth doing it.
After you have refilled it, turn the heater on high and go for a bit of a quiet drive, water level should drop a bit. I have found this to work with mine, the air seems to bleed out OK.
"I married Miss Right. I just didn't know her first name was Always."