Managed to sort this.
The problem was the o-ring under the metering head. I recently replaced this as it had perished, but I used one from a box I had lying around instead of measuring it. It was about 3.6mm thick.
I replaced this with a 2mm one and it appears to have fixed a few things. Thinking about it, the thicker o-ring would have changed the relationship between the airflow plate and the plunger position.
I had to dial the co screw down quite dramatically after that as it was set in relation to the wrong o-ring.
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