Thanks for all the input guys.
It could well be salt at the root of the problem, assuming that the salt was already in the leather. Presumably, the water disolved the salt and was "pushed" up through the wet leather, leaving a concentrated band of salt laden leather halfway up.
I'll give the vinegar a go, any particular vinegar? I don't want to smell like a fish & chip shop...but I suppose most of you wouldn't know what one of those was.
