Simon, just re-read your post. Sounds as if your spares are Birfield. You are lucky to have those, but it could be Tracta, Chobham or Birfield on the vehicle!
It easy enough to dismantle it all. The difficult bit is assembling the hub so that the lips of the two hub seals don't get squashed by the sharp rims of the hub assembly & subsequently leak oil. The hub should be packed with grease, a lot of seals get damaged, I have seen this on Humbers done by owners & because of the vehicle history done in service :roll:. This will happen if you don't have the special tool Sleeve FV252151. I had one manufactured. I can give you the dimensions or how to fake one using two sizes of stock metric tubing. The inner seals are unobtainable, a few years ago I had the last ones that Budge had, but the outer seals were obtained from a bearing company the next day delivery.
Once the hub is back on there is a special tool to be made to hold it in position as you remove the aligning sleeve. But I use a wide jawed welding clamp & holds it ok. The grease in the hub is not adequate lubrication, oil has to be injected through the lubricators on the hub housing until it flows into the other joint mounted in the diff.
Before you take the wheel stations apart. Drain oil out of the diff & see if you get metal particles suggestive of diff failure. If the spare shafts came with the vehicle, one has to wonder why the previous owner didn't replace them himself.