Have put on a bit of weight recently. So I need LESS rebound damping, right?

You need more pressure, and consequently more rebound.

More spring pressure and More rebound damping. – beaten to it by Ben!

Assuming you’ve put more air in your fork then you’ll want more rebound damping.
More air = higher spring rate so more natural rebound force.
Adding some rebound balance it out.
Although in my experience if you’ve gone up 5-10psi you shouldn’t need to change much / far

Magic, thanks all.

As above.
Interesting thing, compression damping is all about what, where and how you ride. Rebound damping is just about the force the spring is generating.