Thanks folks. Little update. To clarify better, I had removed the leather skin from the seatfoam base, but the leather skin has a thin 4mm foam sewed to back of it, which also had some oil stains, so I carefully cut it away, to leave the bare rough underside of the leather. I sprinkled lots of bicarbonate powder onto it, and left for 6 hours. Not much improvement, so oil must be soaked into the leather. I then mixed some becarbonate with vinegar, and brushed on this foamy residue, then let it dry in. I then scrubbed with warmsoapy water, then added some leather food cream to replace the natural oils. Smell seemed to have gone,so I carefully remounted the leather skin the best I could onto my stripped clean cloth seat foam, and congratulated myself on saving the £240 to recover my cloth seat base. The day after today), I went to work, and my car is smelling of oil again. Oil smell seems to be coming out of the leather itself, and prob noticeable, as the car has no air moving through it, when parked up. 3 of those little neutragenia house air freshner tubs now in, but looks like I'm de-skinning the flaming skin again, to try the suggestions above. Think the lesson here is, if you are buying leather, if the back seat base foam, is not a perfect unstained yellow, don't go near it with a bargepool!