Would changing castor have an impact on bump steer as it changes the height of the steer arm relative to the inner tie-rod?

Don't know if I have a clear picture of all this in my mind, but if this is true then could the amount of bump steer be reduced/increased depending on your alignment setting?

Yes, in an extreme case. Going from 0 to 2 degrees positive or negative caster is not going to make much impact on bump steer but going from 0 to 10 degrees positive or negative will.