You will not fail to have noticed that the photo's from the Aero Challenge clearly show that oil is being dropped each lap, as the number of streaks grow as the laps tick by.
After the cars had passed Brooklands on the first lap the marshals were busy debating as to whether there was oil on the track and discussed use of the 'slippery surface flag'. Unfortunately it was only after Sharlie spun on lap two that doubt were overcome and the flags waved.
Take a look at the link below and see if you feel that it might show the culprit.
I think it fair to point out that Sharlie's was not an un forced error, but the result of hitting the oil. (That's £50 you owe me Mrs Goddard)
Bob.
http://sdrv.ms/19nKXaS