Steve,
Thanks for the pointer. I've seen the two pictures of the Caerphilly event in the mpl collection--the first is the famous image of Raymond Mays losing a wheel off his Bugatti, and the second is a view of what is claimed to be the top of Caerphilly Mountain. I've also found a third image on the SandSpeedWales web site (
http://www.automotive-art.com/product.asp?id=GF0010) which shows Leopoldo Alphonso Villa and Malcolm Campbell racing a Sunbeam at the event.
All three photos are difficult to place, though I have found a reference to the race on John Davies' website (
http://freespace.virgin.net/john.davies0/venues.html) which places the course on "the Mountain road". One of the principle routes to Cardiff over Caerphilly Mountain is called Mountain Road, which makes it a fair candidate. There's another bit of evidence which almost clinches it--most records agree that the course was 1194 yards. Using Google Earth, I've measured from the car park at the top of the mountain to a junction on the road just above the town, and the result of my rather crude survey was 1195 yards. Q.E.D. (ish).
My only reservation revolves around that Raymond Mays photograph. Someone has painted a version of it, and the image is somewhat larger than the photos I've seen on the web (
http://www.automotive-art.com/product.asp?id=GF0010). This shows the roofs of houses on the left, which are a little difficult to locate in relation to the course. I still find the evidence of the course distance quite compelling though.