Gav, when you have planked the other side up to the same stage as your picture, carry on the same side with three or four planks......This sometimes creates a slight 'twist' the other way and might help to square the hull up a bit. But if you're happy with the hull, and it doesn't sound as though there is much of a twist....leave well alone.
I hope the above makes some sort of sense...
Then as Derek writes, do alternate sides as you progress with the planking.
Looking good.
Ron