So here is the nearly finished unpainted result, thanks to a little effort over the last couple of evenings as well as on the demo stand. The mini digger is from Wiking, to HO scale but looks fine here, while both wagons are adapted from the 009 society kit. The left-hand wagon has one end removed and the other reduced to 2 planks, and is used to store ramps and timbers for unloading, as well as the drill attachment. I'm not sure the best arrangement to stow these and they are currently loose.
The digger carrying wagon itself is significantly adapted. One end is removed and the end frame tucked further under the floor, while the nearer end is sloped down. The end frame is under the sides, which are cut to a taper, and the removed wedge refitted as a sloping brace to the lower end frame. Under the end a beam is fitted just above track level, to be wedged with a timber during unloading to stop the wagon tilting (a detail taken from the RNAD forklift carrier). The wire loops are supports for the removable ramps, while the deck is reinforced with checker plate. The brake handle has had to be shortened too.
To hold the digger in place I fitted a 2mm diameter, 2mm deep magnet in the wagon deck, and another in a hole drilled in the underside of the digger. These tiny magnets are very strong and hold the digger firmly (it can lift the wagon), but allowing it to be removed. There's a couple of strips of lead under the wagon to make it stable. I suspect the real mini-digger would have a very low centre of gravity, despite appearances.
Stowed and ready to go. This pair of wagons will look right at home in the siding on Hexworthy. I just need to add couplings, and paint the wagons, plus weather them and the digger of course. I'm not sure what colour to paint the wagons - my usual grey, or perhaps a departmental yellow?