Great shot, Colin! Nice field.
I often get strong colour casts after integrating calibrated raw subs in PI (mainly blue). First thing to do is create a small preview selection on an area of background, it doesn't have to be a large preview, a small selection will do. Next, open the Background neutralization tool and select the preview you have created as a reference. Click apply and the colour cast should be balanced.
The result could look ugly at first, just click the auto apply button on the STF (Screen transfer function) tool to reset the artificial stretch. Next thing to do is crop any nasty edges then use DBE to correct any background gradients.
Stick with integrating in PI, it is far more powerful than DSS.
Cheers
Rod