I managed to work out the breather connection is a squeeze and pull removal. So today I uncoupled it. Access is tight from above, and working blind after scoping it out with a torch and mirror.
To blank off the hole in the inlet manifold, which is a short plastic tube, I turned down a short length of rubber hose using my electric drill, and some wet and dry. Once that was a snug fit, I glued a bolt and washer over the end to stop the tube getting inside the manifold.
With regard to the disconnected breather tube, access is so tight that it's almost impossible to do much with it, without removing the inlet manifold, so for now I've left it disconnected and venting to atmosphere. It would be simple to reattach if necessary.
DaveW '05 Red Roadster S1 '16 Yellow (Not the only) Narrow AR GDI Plus 4