I gave your Colour Balance idea a go Alex and it seems to have had the desired affect. I needed to redo a bit of curves after but I'm definitely happier with the background colour now.
Unfortunately the CB did have the effect of a strong clip on the white point end. Might have to work on that a bit. Maybe I'll look into a bit of masking or I could have used the fade option.
The Histo shows before, CB then Curves