Thanks John, that sounds like the stuff. I've got oak galls, rust and vinegar (I guess the rust acts as a mordant?). Haven't got gum arabic or pine resin - what do they do? Is it vital or can I get away without using the resin?
OK - recipe time!
get a ball of wire wool, a dozen or so Galls and a coffee jar of Sarsons Malt Vinegar (or similar).
Burn the wire wool to obtain Iron Oxide (rust) or just pop it into the coffee jar. Half fill the jar with vinegar and allow the vinegar to rust the wire wool and rust it naturally or to mix with the Iron Oxide. Leave until the mix will stain Veg tanned leather to a pitch black...this may take a long time...my current mix is about 12 months old.
Alternatively mix vinegar with Iron tablets from the chemist.......
This mix so far makes a great leather dye for veg tanned leather - but it neads neutralising with bicarb if you use it on leather or the acid from the vinegar will eventually eat the leather and or corode any metal you have in contact with the leather.
Take your Oak Galls (high in Tannin - which veg tanned leather is also) and crush them to a fine powder.
Put the vinegar mix and the Gall powder into a metal pot and simmer together.
This is when you can add the Gum or resin if you want though it is not realy needed for a stain...it makes a better ink though. Resin may "clump" and need more heat to emulsify it. If you use the stain with resin apply it hot.
Test the strength of the stain on scrap as it reduces in the pot - make more than you think you may need and stronger than you think you want it.
Apply the stain with a brush or cloth (less wastage with a brush). Give several coats to build up the depth of colour - it will fade a little when first washed but is theresfter quite robust and long lasting.