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 😄

Values all work, all new rads have had new valves fitted.

Used squeeze offs on the plastic pipe so didn't lose much water at all when fitting.

Yep, did each rad by closing them all and opening one at a time, starting downstairs and moving up. Defo fully cleared the entire system.

Top point is the towel rad in the bathroom, but air seems to find its way to one rad in the master bedroom over the others.

There is an auto air valve on the HW tank, if that helps