My mpg has actually gone up ever since I had my diff replaced by Bell Engineering. All I can put this down to is that the diff was so tight to turn, Austen showed me, he physically couldn't turn it by hand.

My sd4 mpg isnt great, late twenties round town, had a few issues this week on steep hills of dorset being down into 3rd gear at times. Im just wondering what led you to realise that the rear diff was tight? i also have an issue fairly regular with the engine hunting at idle, makes for interesting reversing when its trying to kangaroo backwards.