Cleaning leather...

[Bluestone]

My parents bought a sofa from DFS and one of the kids got a small mark on it. Not sure what it was - grubby fingers probably.
My Mum wiped it over with a clean damp cloth and although the stain came away, it's left a water mark. There's now a patch of darker leather and I just wondered is there any way of getting this off or will trying to remove it just make it worse?
TIA

[deggers316]

we use the standard leather wipes from any supermarket approx £1