Get some detergent powder - the Bio version - and mix it to a paste with a little water. Spread the paste thickly over the oil stain, cover with a plastic sheet and leave for 24 hours.
Scrape most of the paste away, add a little hot water and scrub the remainder well into the surface with a very stiff nylon brush (do not use a wire brush). Scrub using circular motions, and clean an area slightly larger than the stain. Leave the detergent and water on the stain for another few hours and wash off with plenty of water.
If all the stain hasn't disappeared, repeat the process.
Go to Robbie Dysas and purchase their Spot Cleaner bottle (around £6.99) which removes oily marks and stains from brickwork and other similar material. Worked a treat on my paved drive, best to apply neat, scrub in with hard brush, leave for around an hour and wash away with boiling hot water and scrubbing brush. Hope that helps.
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