Firstly, I need to mention that I'm in the U.S., because a rather benign ingredient in a simple process that really works may not be available in the U.K. So... power wash the garage floor after first applying a
non[i flammable cleaner ( don't use gasoline... ooops. I mean petrol) with a good, stiff broom. After the power wash has thoroughly dried, apply enough [i]Muratic Acid] to wet the oily areas and brush again with your broom. Immediately pressure wash the residue.
Muratic acid is pictured here:
http://www.lowes.com/pd_58558-78-GKGM75006C_4294729410__?productId=3220089&Ns=p_product_avg_rating%7C1&pl=1&currentURL=%3FNs%3Dp_product_avg_rating%7C1&facetInfo=
Wear old shoes, gloves and eye protection. It's not that dangerous nor is it particularly long lasting once it's diluted with the water from power washing... but it does leave the concrete completely clean and whitish...