I've seen a defender with leaky front hubs, and that manifests itself as oil on the wheels. It only appears after a long drive, and little or nothing escapes when the vehicle is still. The oil is (I think) in the swivel pin housing, and can be replaced with a special grease. The defender gets through the test no problems.
If your diff/hub is leaking, I'd be inclined to check the level in them anyway, to see how much you've lost.
Try cleaning up the oily bits, go for a drive and see if any more oil escapes. If you have it clean at the MOT, they can't see the leak.
However, if a lot is coming out, you want to get it fixed anyway.