Sounds like you have an air lock in the heater matrix.
If you’ve flushed it then it shouldn’t be blocked… so probably just need a few drives / heat cycles to get the air out
When you see the temp gauge starting to rise, can you see a drop in coolant level in the expansion tank? If you gently crack open the cap (when safe to do so) the air should escape and coolant level return to normal
Update- So all was going well until I went for a longer drive. Coolant spewing out of expansion bottle top. During the head change I have changed the expansion bottle, thermostat and temp sensor.
I have no heat in the matrix and car overheating. Since then I have flushed the matrix, flushed the radiator and replaced the thermostat and heat sensor with my original ones. I have ordered a new expansion bottle cap bmw blue one. Currently still no heat in the matrix and this evening the heat indiction in the dash was higher than normal, so I switched off. My next option is to buy a new matrix, new heater valve and hope that does the trick.
Edited by jamesmpi, 26 May 2023 - 08:55 PM.