Our models have different engines on this side of the pond, but the cooling system is likely "similar".
Ours are self bleeding, with some small hoses (tiny plastic things) leading back to the reservoir bottle to purge any trapped air & bubbles that may develop.
Checking those for any damage (pinched, kinked), then removing & blowing back through them to check for obstruction (gently, like with your mouth) might find/cure your problem. (small obstruction would be blown out)
SOME air bubbles develop during normal operation, and when working right the system takes care of itself.
Hope this helps, Cheers!