Thanks for the tip!
For future reference, if you get a water stain on your new Mulberry, don't panic! This happened to me the second day after I got a brand new tooled Bayswater, and in trying to get it out I made it worse. I thought I would cry, but now I've just accepted it as part of the character of the bag.
The Darwin leather is of a type that if you apply any creams or cleansers to it it will make it darker. So, while you or a cobbler may be able to treat the leather to make the water spot less noticeable, the color of your bag will also darken a bit. This is part of the natural aging process for the leather anyway so perhaps you won't mind. However, I was disappointed because I did want my bag to look "brand new" for a while and develop its patina slowly.
The best thing to do is to apply the Collonil Waterstop spray product in neutral as recommended by Mulberry before you take your bags out for use. This will help repel water and prevent stains. It will darken your bag slightly at first but most of it will disappear when it dries and at least the color will be even.