Mark,
Seeing as you don't have the time scale to order in, the big sheds stock Thompson's oil stain remover, which I know a couple of people of here have used and have said it worked for them, but if you have time for the client to wait then Resiblock OR is about the best that is around at the present time IMO.
OT8 is also good, I use this as a second application if the stain is stubborn and the Resiblock OR didn't quite get it first time.
All the above are fine on concrete slabs-paving