The diff is in - after a struggle!

 It took us about 4 hours to get the differential located in the chassis whereas Caterham say it should take 45 minutes! Probably next time (there won't be a next time!) will be quicker. The problem was you could get either the top bolts in or the bottom bolts in but not at the same time because the chassis holes were not in sympathy with the differential holes.
In the picture above you will see the metal we had to grind off the top bolt (top middle of picture) to make a point so that we could drift it through the width of the diff (bottom picture).

Anyway this now opens the door for the next stage which is getting all of the rear suspension on.