I spotted the patch of diesel too late; I was fully commited to the bend anyway. As my rear tyre went slippy slidey I thought: " Oh Fuck - here we go" Fortunately, it was a small patch, about 1 m sq. Next thought was: " It'll be ok though as there (for once) is no car following 3 ft behind me, I will just do the standard lowside getoff" Rear wheel found the dry tarmac, and with a twitch things were back to normal. I hate Diesel.