OK, I think I may have figured out what’s happened here, and this might well be a bug: I upgraded the original project from 4.14 to 4.15, when some changes seem to have been made to post processing. Something must have been broken behind the scenes though, making the control (which now has a new name) not work.
I realised this because I was looking over this thread about bloom/emmissives in the new post processing and realised that in my version of the project which is an upgrade from 4.14 ->4.15 I’m actually getting the old bloom effect. But if I create a new 4.15 project from scratch, and then migrate the level over to the new project, I get the new (and unfortunately uglier) bloom effects, as well as suddenly getting the Chromatic Aberation control working.
So something in the transforming of a project from 4.14 -> 4.15 something went wrong in relation to the post processing.
So now if I can just find a way to have chromatic aberation AND the lovely old bloom effect, but still be in 4.15…