The temperature no longer hits 100C, having flushed the radiator again, but I will try putting my other thermostat. Keep you posted on that.
While I'm at it I think I'll also put in a new radiator since the one currently in it was left for a while with old antifreeze in it and not driven. I'm guessing there's some rust on the inside.

When you say to back fill it through the port on the head, you are referring to the top port directly to the front left (as looking at the engine from the front of the car) of the valve cover? The one that the top radiator hose plugs into?

If so, then, yes, this was filled totally. I did not think to measure the amount of coolant I put in, but I will certainly do this when I change the thermostat.