The heater matrix is a pain to change
1st thing you should know is the temprature sensor for the ECU and the temp gauge is on the metal of the head, it reads the head temp even if the water runs dry
2nd thing is that if the heater matrix gets blocked the water can still circulate round the system when the main thermostat opens - it is an alternative route
Double check the system is bled and has no air locks, it is difficult to bleed and the correct procedure must be used (i forget what it is exactly but its a lot of hassle )
Did you fit a brand new waterpump? was the gasket between the pump and plock put in the right way? the pump is driven from a pully off the camshaft through the power steering pump - were the splines ok on that?
As a temporary measure - take the main thermostat out and run the car, see what happens
the main thermostat is at the end of the head, near the battery, its housing is held on by 3 bolts,there is another, smaller thermostat for the oil cooler coming off the waterpump housing
I took the main stat out to run it without it in but water just ran out quickly, I have tested that stat and it's fine. If there was an air lock surely the problem would have appeared before and not after 2 months and a thousand miles?
I did fit a brand new water pump but the gasket to the head was torn so I used some gasket seal as well. There are no leaks around it.
I have removed the bleed screw on the pump and water rises out of that so presume it's working, I haven't check the shaft/blades yet as it's a pain to remove them but that might be where to go next?
it depends - if its lower than the resovoir it may syphon out even if the pump is kaput
An option is to leave the waterpump on, with its shaft/ blades mounted, and just remove the plastic side cover
Alternatively, you could somehow check that the water is circulating,