The rear wishbone that had been 3mm too narrow has now been widened back to normal size. A long M12 bolt, threaded through the opening with a nut, washer and fender washer at each end was used to push the wishbone opening from an 87mm gap out to approximately 100mm. At that point, the welded plate on the top of the wishbone came into affect and started to limit the movement and the arms begin to twist slightly so that was time to back off a little.
I used the old, worn thrust washers to protect the pins on the new bushes and then rotated the nuts in opposite directions to against the washers which pushed against the bushes, which pushed out the arms of the wishbone. Releasing the tension by undoing the nuts left the wishbone widened out to 92mm which will be enough to allow easy re-assembly of the wishbone outer bearings and hub.