deffo sounds like an airlock, when you were adding coolant were you squeezing hoses while the expansion tank cap was off, this dispels air , sounds as if you have just filled it up with coolant and put the cap on and not got rid of air
Ive taken lowest pipe off and drained the water out then put 50/50 water anti freeze put pipe back on filled up ....but seems to of only taken 2liter witch cant be right .... then started car with cap off and left heaters on till fan kicked in then put cap on now heating only cold
When doing a coolant change you need to loosen,do not remove all the bleed screws you can find
and then as you are putting the fresh coolant in close the bleed screws in turn that start to leak out the coolant.
When you have closed the last bleed screw and filled the expansion bottle to proper level start the car and put heating on full and rev the car in quick bursts up to 3000rpm
with the coolant cap still off.You may see some coolant splatter out of the bottle but that's ok.
After the revving you may see that the coolant level has dropped so top up again.
Your system will be free of air when you see the coolant returning to the expansion bottle via a little hole near the top of the expansion tank.