make a duplicate of the background, convert that copy's blend mode to 'multiply' and then add an adjustment layer of 'brightness/contrast' and raise the lightness a bit to taste. it shld hide most of the banding and lose no data through 'noise reduction' (which happens sometimes). you may lose a little luminance if you only adjust a tiny bit and you may still see the banding if you go too high, so, try to find a nice median. and, you can also do a bit of dodge and burn if necessary.
if that's not good enough, you could also try the fft filter and see how that goes.
oh, and welcome to RetouchPRO
