Hi again all. I'm the original poster, and as I just noticed that there's still a bit of life in this thread, I decided to post an update. I have had to take a long break from leatherworking, and have only just started up again, but I have just dyed for the first time, and I must say that I am very pleased with the result. For those who were worried, I can say that I used nothing but water and the dye itself - I might use oxalic acid if I ever need to dye something really evenly(sp?), but I like the (very) minor variations that were left after dying with no cleaning beforehand.
However, the reason that I'm updating is also that I were in a leather shop the other day, and had a strange experience. The shop is one of those old, dedicated, 'existed since 18xx' shops, so they know what they're doing, or so you'd think. Anyway, I bought some dye and the lady at the desk asked me if I had everything else needed for dying. I asked her to clarify, thinking that she might have a tip or two, and the first thing she mentioned was acetone for cleaning... I didn't buy any, but still, I were surprised after what I have read here. Maybe it's just some regional thing?