So I thought I'd update the thread a little.
I have spent a bit of time trying different products to see if I can shift that bottom lumbar mark.
In short; I can't.
I've tried going back over it numerous times with my AutoGlym leather shampoo, used a microfibre cloth, medium brush, steam cleaner, you name it, but it actually got a little worse.
I followed AMMO NYC's leather cleaning guide without any luck:
I then tried with sugar soap which actually had a really good effect on the rest of the leather, which I then followed up with autoglym leather cleaner, and cream.
In a nutshell, I'm just very disappointed on how badly the seats wear.
I plan to try a warranty claim with BMW as my car is still under manufacturers warranty and there is simply no reason that after just 60k km's, the seat should look this way.
My next car will either have alcatara seats again (really do prefer these), or black leather seats. Oyster just looks terrible after 5 minutes of use (jean stains and these hideous lumbar marks).