Don't shoot me as I know this write up doesn't concern a BMW as I did it on my C70 - but its a common affliction for most cars with leather interiors so i thought it might help!
I finally got round to sorting my interior out - so thought I'd do a quick write up.
I ordered my Liquid Leather Scuff Master treatment kit from http://www.liquidleather.com - in the order form or if you call you specify the make, model and year of car as well as colour of leather. They then send you out the kit pre-mixed to a sample they hold on file. If you have any reservations about colour matching then you can always send them a small sample of your leather and they'll match it for you - although in the kit you do get 2 small bottles of toners to make up for any slight variations.
Here's a picture of what you get in the kit as well as some other items you might find useful for this job:
GT12 Liquid Leather Cleaner
GT11 Liquid Leather Conditioner
Scuff Master Touch Up Kit (contains 1 x bottle of premixed dye, 1 x light toner, 1 x dark toner, 1 x gloss enhancer and 2 x sponge applicators)
In addition to the supplied kit you might also want to consider the following:
Rubber/latex gloves (if it can dye leather I'm pretty sure it can dye skin! lol!!)
Small artist paintbrush or cocktail stick (for application of neat dye)
Small plastic bowl
Hoover your interior and seats
Clean all of your leather interior thoroughly with the supplied GT12 Liquid Leather Cleaner. You can use a small nail brush or old toothbrush to agitate the cleaner on stubborn areas and then make sure you clean off all the residue and leave to dry.
Once dry - wipe down the seats/interior with a mild solvent like methylated spirits and dry.
Take your pre mixed Scuff Master dye and apply neat to a small crack/scuff with your cocktail stick/brush and wait for it to dry. Once dry inspect to see how accurate colour match is and add toner to adjust accordingly. Once you are happy with the colour match proceed to paint in the remaining cracks - I found that this worked best by working along the creases and not try to over paint areas that shouldn't have dye applied. The same applies to scuffs - apply a slightly diluted mix (10-20% water) to the scuffs with the sponge applicator.
Leave the dye to dry for around 1 minute then wipe off with a damp cloth (work along the direction of the cracks/scuff) and repeat Step 4 if necessary. Once you are happy all of the scuffs/cracks have been dyed then move onto Step 6.
Prepare a colour wash for the treated seat - for my first colour wash I mixed around 80% dye with 20% water and applied this over the seat. Again I let this dry and then rubbed down with a damp cloth. I was quite happy with the match/balance of the colour at this stage so mixed my final colour wash which consisted of around 20% dye and 80% water and added a little drop of the gloss enhancer I received with the kit. This colour wash was then applied to the seat and I left it to dry (24 hours).
Take the GT11 Leather Conditioner and work this into your leather until it is completely absorbed. The aim here is to apply an even coat and make sure you work the conditioner completely into the leather. Areas which are particularly dry may require repeated application - just be patient and make sure it is worked in properly.
Job done! - with any luck your interior should look something like mine!