Well, I managed to pursuade a garage to get my car up on their lift (very carefully) and sadly, no stones, no nails, and nothing around the prop/drive shaft so now I'm stumped. The guy at the garage had a look and said he couldn't be sure and not to hold him to it but he thought the collar on the right side of the LSD wasn't where it should be and thought there was some play in it. He also said that maybe a bearing is wearing or worn out in there. I will phone Nissan on Monday and book it in......It's beginning to sound like it's going to get expensive very quickly!