Robin - This is a diagram of the nozzle/injection entry on the rear of the mold. The 'small internal cone' I think is hot metal cone that keeps the plastic from setting in the entrance to the injector point. Is this what you call a plasticiser?

I think you are right that this is not so critical to get all the angles and rads just so. I think the only critical point would be the depth that the hot cone pokes into the mold specifically the 1.1mm diameter hole that feeds the mold cavitiy. If it goes in too far it will block the mold and if its too far away it might allow the plastic to set?