It is a Diatto with a 15.9-litre engine.
Yes, and another point for you.
I can't find anything about this car, and my puzzle was based purely on what the caption claimed. Diatto had built Clement-Bayard cars until 1908, and then had financial problems for a few years. The date is too early for it to be a WW1 aero engine, and if Diatto were in money trouble, it seems unlikely they would have built this and transported it all the way to Brooklands. It's a bit of an enigma - does anyone have a history of Diatto which will enlighten us?