It could be the natural color of the leather shining through. If that is the case then cleaner will make it worse. I've had good luck on white bags using zit cream (benzoyl peroxide 10%) and putting the bag in the sun. It is basically bleach. It turns them very white so if your bag is off-white, it might brighten it too much.
Thanks love. It's definitely some sort of stain. I will have to try the cream!