Plea for information: The Egham Races

One unexpected thing I found at the weekend was in the gallery's kitchen. On the wall is a copy of a plan of the Runnymede Race Course. It turns out that before Kempton Park was built, horse racing took place on the mead and apparently only moved on thanks to gangs of pickpockets from London.

Thanks to the Your Paintings archive, I've found one painting of the races. (I don't think I've mentioned Your Paintings before - its an astounding resource on the BBC website which has every single painting in every single public collection in the UK.) The painting is by John Spoade and titled "William IV Arriving at Long Mede, Runnymede at the Point of the One Mile Winning Post."

I'm going to transpose the map onto a modern one so I can trace the course as it is now and see what other tidbits I can find, but being lazy I was also hoping there might be people reading this who already have information or documents.

Any help will be appreciated, either via comments or email.