I would do what I did, bite the bullet and fit a header tank. Our primatic repeatedly lost the air lock and mixed the water leading to bad corrosion of the heating system.
I am busy unblocking my microbore pipes. I discovered that the blockage is always near the beginning where the microbore comes off the manifold connected to the larger bore supply pipe. I presume the temperature drops and possibly there's also a stagnant reservoir there. I was able to unsolder...
