More info on the D/F trailer
here - select army, then vehicles., then DF Trailer
I would assume that 2 trailers are required to "triangulate" a signal's position - maybe the Bedford was used to find a signal then the trailers would be sent out to fix the position, both being towed by something smaller (they're only 15cwt. trailers).
Also there's probably some information on the Trux website about this
Noel