You need to complain right away.
Forgetting the issue that the mortar is different and may well be weaker mix, but not necessarily, this is just plain wrong and unacceptable.
Whilst bricks do vary from each delivery and batch to batch, they should be mixed so this banding does not occur. However, I am of the opinion as it is so pronounced, that a different brick has been used altogether, probably due to supply problems with the original specified brick.
Final note of caution, Barratt will fight you over this and offer tinting. This is never as good as if the house had been built properly. If you accept tinting do so on the written strict proviso, that if you are unhappy with the result, you expect the gables to be taken down and re built in better matching bricks and mortar.
Finally, please get you home professionally snagged and inspected before you move in.
I've put the picture on Twitter, as it is so bad.