I've re-jigged it now based on BB's comments and having read the "Artistic Lens" thread he linked to above...

...also tried to separate out the variables.

The three maths nodes along the bottom row are (from left to right) meant to be controlling IOR-based "magnification boost factor" (just set to a factor of 1 there - no boost), material IOR (e.g. set to 1.523, Crown Glass), IOR variance amount (for the Chromatic Aberration - e.g. 5% variance either way, I think, in the example there).

Also added in a blender node to control the diffuse value numerically.

Guess I could just plug it all into Diffuse master channel, instead of Alt_Diffuse, and use the built in Diffuse Value... what's the difference or benefit with using the Alt_Diffuse, over doing that?