I think (as the owner of a D2 S8 with a failed 'box), I am going to agree with IT/Ian on this one: I can't think of many owners on this forum with a D2 that has survived much beyond the mileages that Ian's listed, and as I would argue that this forum is frequented by the majority of D2 enthusiast owners, I think it's fair to conclude that these boxes did have this inherent problem.
The good news is that the boxes can be rebuilt, and a failure avoided if the oil service is carried out regularly (which I wish I'd done). Either way, I've now got a fully working D2 with a rebuilt box which is better than the original one, so it doesn't have to be terminal..