At a guess the old radiator cap was dicky and not letting the system get up to pressure, the rad came with a new 1.1bar cap, now when I get up to temp, I get this:

It's quite the leak, it emptied my overflow bottle in about 20 minutes of idling - initially I thought it'd burped up a bubble and sucked the clt out of the overflow bottle, but no, it's just pumping it out.
Now we wait for the car to cool down....
