Baz,
Not used a jig for a Bedford cab, but just done some structural work on a Fordson cab, it did not require removing from the chassis in this case, but I would strongly advise that if it is severely corroded in the lower area, to build a space frame of sorts, within the cab, before removing it, bolting it to existing points of good strength, in order to preserve its integrity. Once removed, it can soon distort then when you cut out the rot and weld new metal in, it may not align again, doors may not fit, etc. If this framework were done right, it may be possible to rotate the cab, as on a spit, with the axis right through the middle of the cab, side to side. This way you will be able to weld at a convenient position.
Just some quick thoughts.........
Richard