Thank you - that's very helpful!

If you try it within the infusion, you will likely see a heavy print through on the mould side where the iron bar is. So if it's a cosmetic part, bond it afterwards. the bonding could be made via infusion, because the outer layer is already hardened.
I suggest your c.f. part is made from epoxy, so you should use epoxy for the second step as well, because laminating polyester doesn't bond well to epoxy. You should use a layer of glass around the steel, because c.f. are conductive and will disintegrate the iron over time through electro chemical reactions. After the glass layer, you could use whatever you prefer.

Can anyone suggest the best method?

Simple question I hope!

I want to bond a steel bar into the part I am creating.

Should I do this as part of the normal vacuum lay up process or let the part first cure then add the steel in a second (non vac) lay up operation? If so should I bond it in using a polyester glass fibre lay up or another cf layup?

Hope that's clear!

Thanks