Hi Bjorn, I did ask the owner's son for more info, nothing as yet.
I received an e-mail from the son yesterday, but was going to wait until he had contacted his dad for more info on dates. Meanwhile I can tell you that he believes it was converted from an ice-cream delivery truck (wholesale I would imagine, rather than a dispensing van), and has channels for the car over the wheel-arches, and the loading ramps fitted under these. The body is mainly alloy, and therefore in reasonable nick, but the rear valance is steel, and has rotted. The roof is fabric over plywood, and as you can imagine, there's not much left. He believes that the engine could be made to run with only a minimum of work.
The history we have ascertained so far suggests that it probably does not have great historical significance, but it's interesting all the same. It's far too early to enquire whether dad would sell IMO. The practicality of using an old truck for historic meetings has been discussed on this thread before - it'll look nice in the paddock, but who wants to drive 100 miles at 40 mph? (I'm reading Tony Robinson's biography at the moment, and apparently the Bruce Halford coach had higher gearing than usual for such a vehicle, but still maxed-out at 38 mph!