I think you will find an interesting article by David Fletcher in the Vintage Commercial Vehicle Magazine, Volume 5, no. 23, November / December 1989 regarding Diplock / Pedrail. Richard Peskett.
Duncan Crow wrote a few lines about the pedrail too, in Armoured Fighting Vehicles in Profile, vol.1, some pictures included too. It seems that Churchill was impressed after a civilian Pedrail was brought to his attention, ordering 12 Pedrails. But tests showed the inadequacy, the vehicle being too long and the tracks vulnerable, or not strong enough for bigger sizes. At the end no military Pedrail was built, orders were cancelled. At the very end some plans were worked out, to build a flame-throwing version which was never taken in production too.