Thanks for your assistance. I'm going to go with my theory of it needing more inhibitor and add another litre of the same stuff I put in it before.

The pressure of the system is at 1.5 bar cold so that's all good.

Glad it's nothing obvious. Just puzzled why it was ok before with less... Weird!

If it still does it, I'll try a full drain and refill. Then call a plumber 😄