Nice video of the Ruston and the BE moving under their own power. Shame the guy driving the Ruston doesn't know how to drive it! In fact he is doing the two things you shouldn't do when driving one! Namely, presetting the throttle at too higher revs and driving it with the (speed) gear lever half in!
Those early Lister engined Rustons had very slow revving engines anyway, much slower than the later Ruston made engines. The Listers ran at 600 rpm (later up rated to 720 rpm).
Incidently, it is a 18/21hp class not a 21DL. The DL classes were introduced quite a bit later.