Thanks Belly_up,
Once again for the people in the cheap seats - you reckon I should block off the line between the plenum and the intake-side rocker cover (I'd remove the valve and replace it with a brass plug).
I should then run one line from each of the rocker covers to the two inlets on the top of the catch can. I should then assess whether the can has filter material in it, if it does, I can plumb back to the inlet piping, if it doesn't I should put a filter in it.
Does that sound like what you mean?