Not sure how this one pans out. When they are in season, they can be shot on a Sunday in Certain Counties of England and Wales anyway, but when they are just on the General License as a pest, I can't see a Sunday being out of bounds, just the same as it isn't for other pest species.
Don't forget that although they are classed as pests, they are a Wildfowl species and still need to be shot with non-toxic shot regardless of the time of year, which unfortunately means no Lead in your bang stick
I'll see what I can dig up.